Secret Prior Art in the USPTO and EPO: Earlier-Filed, Later-Published Patent Applications in Comparative Perspective

Executive Summary: This article compares how the USPTO and EPO treat “secret prior art”—earlier-filed patent applications that were not public when a later application was filed but later acquire prior-art effect after publication. In the United States, AIA § 102(a)(2) treats qualifying U.S. patent documents, including certain PCT publications designating the United States, as prior art as of their effective filing dates if they name another inventor; such references may be used for both anticipation and obviousness, but AIA § 102(b)(2) provides important inventor-origin, prior public disclosure, common-ownership, and joint-research-agreement exceptions. In Europe, EPC Article 54(3) similarly captures earlier-filed, later-published European and qualifying Euro-PCT applications, but Article 56 limits their effect to novelty only, with no U.S.-style common-ownership escape from self-collision. The article’s central claim is that the U.S. system broadly incorporates secret filings into the prior-art universe and then mitigates harshness through statutory exceptions, while the EPC sharply limits secret filings to novelty but relies on priority, direct-and-unambiguous disclosure, amendment practice, and narrowly available disclaimers rather than ownership-based relief.

I.                Introduction 

A common patentability problem arises when a patent application is filed before the application under examination, but is not published until after the later application’s filing date. Consider a simple chronology. Applicant A files Application A in January. Applicant B files Application B in June. Application A publishes in July. When B filed in June, A’s application was still confidential and could not have been found in a prior-art search. Yet once A publishes, both the United States Patent and Trademark Office and the European Patent Office may treat A as prior art against B.

This article uses “secret prior art” as shorthand for earlier-filed patent applications that were not publicly available when the later application was filed but later acquire prior-art effect by operation of patent statute. The term is not itself the operative statutory phrase in either system; in U.S. practice, the principal modern category is AIA 35 U.S.C. § 102(a)(2) prior art, while in EPO practice the comparable category is Article 54(3) EPC prior art.

The systems assign different consequences to the same fact pattern. In the United States, if Application A becomes a qualifying U.S. patent document, names another inventor, and was effectively filed before B’s effective filing date, it may be prior art under AIA 35 U.S.C. § 102(a)(2), subject to the exceptions in § 102(b)(2). See 35 U.S.C. § 102(a)(2); MPEP § 2154.01 (9th ed. Rev. 01.2024, Nov. 2024). That prior art may support both anticipation and obviousness. See 35 U.S.C. §§ 102(a)(2), 103; MPEP § 717.01(a) (9th ed. Rev. 01.2024, Nov. 2024).

In Europe, if Application A is an earlier European patent application, or a qualifying Euro-PCT application, Article 54(3) EPC may place the content of A as filed into the state of the art against B. EPC art. 54(3). But Article 56 EPC excludes Article 54(3) documents from the inventive-step analysis. EPC art. 56. The Article 54(3) reference is therefore novelty-only prior art.

The policy divergence is central. The U.S. system treats secret patent filings as part of the general prior-art universe, then mitigates harshness through statutory exceptions. The EPC takes the opposite approach: it sharply limits the use of secret filings to novelty, but offers no ownership-based escape from self-collision.

The practical contrast is this: U.S. secret prior art has broader substantive reach because it may support obviousness, but U.S. law contains inventor-origin and common-ownership exceptions. EPO secret prior art has narrower substantive reach because it is novelty-only, but it is more rigid in same-owner portfolio situations because the EPC has no direct counterpart to AIA § 102(b)(2)(C).

II.          Comparison at a Glance

Issue	USPTO / AIA	EPO / EPC
Eligible documents	U.S. patents, U.S. patent application publications, and WIPO-published PCT applications designating the United States. 35 U.S.C. § 102(a)(2); MPEP § 2154.01.	European patent applications as filed, including qualifying Euro-PCT applications. EPC arts. 54(3), 153; EPC r. 165.
Prior-art date	The date on which the reference was “effectively filed” for the subject matter relied upon. 35 U.S.C. § 102(d).	The earlier filing date or valid priority date of the earlier European application. EPC arts. 54(3), 89.
Novelty effect	Yes. A qualifying § 102(a)(2) reference may anticipate. 35 U.S.C. § 102(a)(2).	Yes. Article 54(3) is novelty prior art. EPC art. 54(3).
Obviousness / inventive-step effect	Yes. § 102(a)(2) prior art may support a § 103 obviousness rejection. 35 U.S.C. § 103; Hazeltine Rsch., Inc. v. Brenner, 382 U.S. 252, 254–56 (1965).	No. Article 54(3) documents are not considered for inventive step. EPC art. 56.
Foreign / PCT treatment	A WIPO-published PCT application designating the United States may qualify regardless of publication language or U.S. national-stage entry. MPEP § 2154.01(a).	A Euro-PCT application can have Article 54(3) effect only if the Rule 159(1)(c) filing fee is paid and, where required, a translation is supplied. EPC r. 165; EPO Guidelines A-XIII, 9.1.3 (2026).
Common-ownership escape	Available under AIA § 102(b)(2)(C), subject to conditions; joint research agreement treatment may also apply under § 102(c).	No direct common-ownership exception to Article 54(3).
Priority battleground	Whether the earlier application to which priority or benefit is claimed describes the subject matter relied upon. 35 U.S.C. § 102(d).	Whether priority is valid for the relevant subject matter under the “same invention” standard. G 2/98, Requirement for Claiming Priority of the “Same Invention,” 2001 O.J. E.P.O. 413.
Typical response strategy	Challenge document eligibility, effective filing date, inventive entity, and § 102(b)(2) exceptions; consider Rule 1.130 declaration practice, common ownership, claim amendment, and double-patenting strategy.	Challenge direct-and-unambiguous disclosure, priority entitlement, Euro-PCT status, and novelty; consider targeted amendment or a tightly limited disclaimer.

III.      The U.S. Framework: AIA Text First, Pre-AIA Cases as Background

For current AIA applications, the starting point is the statute. Section 102(a)(2) provides that a person is not entitled to a patent if the claimed invention was described in a patent issued under § 151, or in an application for patent published or deemed published under § 122(b), in which the patent or application names another inventor and was effectively filed before the effective filing date of the claimed invention. 35 U.S.C. § 102(a)(2). The USPTO groups these references as “U.S. patent documents”: U.S. patents, U.S. patent application publications, and certain WIPO-published PCT applications. MPEP § 2154.01.

The reference’s publication date is not the controlling prior-art date. The reference must, of course, have become a patent or published application in order to be applied in the ordinary way, but the statutory question is whether it was “effectively filed” before the effective filing date of the claimed invention. 35 U.S.C. §§ 102(a)(2), 102(d). Section 102(d) supplies the effective-filing-date rule: a U.S. patent document is effectively filed as of its actual filing date or, where applicable, the filing date of an earlier priority or benefit application that describes the subject matter relied upon. 35 U.S.C. § 102(d); MPEP § 2154.01(b).

This AIA rule differs materially from pre-AIA § 102(e). Under the pre-AIA Hilmer doctrine, foreign priority did not generally supply the § 102(e) prior-art date for a U.S. patent reference. In re Hilmer, 359 F.2d 859, 878–79 (C.C.P.A. 1966). Under AIA § 102(d), by contrast, the USPTO treats a U.S. patent document as effectively filed as of the earliest priority or benefit application, including a foreign or international application, that describes the subject matter relied upon. 35 U.S.C. § 102(d); MPEP § 2154.01(b). Hilmer remains important as historical background, but not as the operative rule for AIA applications.

The phrase “names another inventor” is a high-value practical limitation, but it should not be read narrowly. Current USPTO guidance states that any difference in inventive entity between the earlier U.S. patent document and the later application satisfies the “names another inventor” requirement. MPEP § 2154.01(c). Thus, where both documents name joint inventors, a single different joint inventor is enough; overlapping inventorship does not prevent § 102(a)(2) prior-art status unless a § 102(b)(2) exception applies. Id. Pre-AIA “by another” cases such as In re Land, 368 F.2d 866, 877–79 (C.C.P.A. 1966), remain useful background, but current AIA analysis turns on the statutory phrase “names another inventor” and the USPTO’s guidance in MPEP § 2154.01(c).

The historical foundation for U.S. secret prior art is Alexander Milburn Co. v. Davis-Bournonville Co., 270 U.S. 390 (1926). Milburn is not the operative AIA statute, but it explains the policy that later Patent Office publication should not let a second filer obtain a patent on subject matter already disclosed in an earlier-filed application. Id. at 399–401. In Milburn, the Supreme Court held that an earlier-filed application could defeat a later patent where the earlier application adequately disclosed the later-claimed invention, even though the earlier application issued later and did not claim the subject matter. Id.

The pre-AIA obviousness foundation is Hazeltine Research, Inc. v. Brenner, 382 U.S. 252 (1965). There, the Supreme Court held that an earlier application pending in the Patent Office when the later application was filed could be prior art for purposes of § 103 obviousness. Id. at 254–56. Although Hazeltine predates the AIA, its central practical lesson survives under the AIA because § 102(a)(2) references are part of the U.S. prior-art universe and may be used in § 103 obviousness analysis. See 35 U.S.C. §§ 102(a)(2), 103; MPEP § 717.01(a).

The pre-AIA priority cases also require careful labeling. In Dynamic Drinkware, LLC v. National Graphics, Inc., the Federal Circuit held, in a pre-AIA § 102(e) setting, that a reference patent was not entitled to a provisional application’s filing date unless the provisional provided written-description support for the reference patent’s claims. 800 F.3d 1375, 1378–82 (Fed. Cir. 2015). In In re Riggs, the Federal Circuit added that, for the provisional date to apply in a pre-AIA § 102(e) rejection, the specific portions of the reference relied upon as prior art also must have written-description support in the provisional application. 131 F.4th 1377, 1384–85 (Fed. Cir. 2025). Although these cases arise in pre-AIA settings, their subject-matter-specific priority logic parallels the AIA’s § 102(d) inquiry, but the statutory analysis should begin with the AIA text.

Lynk Labs is best understood narrowly. It is not an AIA § 102(a)(2) decision; it arose under pre-AIA § 102(e)(1) in the IPR context. Lynk Labs, Inc. v. Samsung Elecs. Co., 125 F.4th 1120, 1124–26 (Fed. Cir. 2025), cert. denied, No. 25-308 (U.S. Mar. 9, 2026). Its relevance is that the Federal Circuit held that a published patent application may be prior art as of its filing date even though it was not publicly accessible until later publication. Id. at 1130–33. The case therefore bolsters the “secret springing prior art” concept, but it should not be read as a direct interpretation of AIA § 102(a)(2).

IV.       U.S. Exceptions: Broad Prior-Art Effect, but Statutory Safety Valves

The breadth of AIA § 102(a)(2) is tempered by § 102(b)(2). The first exception removes subject matter that was obtained directly or indirectly from the inventor or a joint inventor. 35 U.S.C. § 102(b)(2)(A). This is the inventor-origin exception and is often raised through declaration practice under 37 C.F.R. § 1.130(a). See MPEP §§ 717.01(a), 2155.01.

The second exception removes subject matter that had been publicly disclosed by the inventor, a joint inventor, or another who obtained the subject matter from the inventor or joint inventor before the effective filing date of the intervening § 102(a)(2) reference. 35 U.S.C. § 102(b)(2)(B). The Federal Circuit’s decision in Sanho Corp. v. Kaijet Technology International Ltd. is a cautionary example. 108 F.4th 1376 (Fed. Cir. 2024). The court held that a private sale did not publicly disclose the subject matter for purposes of § 102(b)(2)(B), even though there was no confidentiality agreement. Id. at 1383–86. The exception requires a true public disclosure of the relevant subject matter, not merely an inventor-origin commercial transaction.

The third exception is the common-ownership exception. Section 102(b)(2)(C) provides that subject matter otherwise qualifying under § 102(a)(2) is not prior art if, not later than the effective filing date of the claimed invention, the subject matter disclosed and the claimed invention were owned by the same person or subject to an obligation of assignment to the same person. 35 U.S.C. § 102(b)(2)(C). Section 102(c) extends related treatment to certain joint research agreements. 35 U.S.C. § 102(c). The USPTO’s guidance makes clear that a proper common-ownership or joint-research-agreement submission may remove the subject matter as § 102(a)(2) prior art, including where the rejection is for anticipation or obviousness. MPEP § 717.02(a).

The common-ownership exception is not a cure-all. It applies only to disclosures used under § 102(a)(2), not to public prior art under § 102(a)(1). 35 U.S.C. § 102(b)(2)(C); MPEP § 717.02(a). Nor does it remove double-patenting issues. MPEP § 717.02(a). A commonly owned earlier application may cease to be § 102(a)(2) prior art, but it may still require statutory or nonstatutory double-patenting analysis and, where appropriate, terminal-disclaimer strategy.

V.          The EPO Framework: Article 54(3) as Novelty-Only Prior Art

Article 54 EPC distinguishes public prior art from earlier European patent filings that were not yet public. Article 54(2) defines the ordinary state of the art as everything made available to the public before the filing date of the European application. EPC art. 54(2). Article 54(3) adds a special category: the content of European patent applications as filed, whose filing dates are earlier than the relevant date of the application under examination and that were published on or after that later date. EPC art. 54(3).

Article 56 EPC supplies the limiting principle. It provides that Article 54(3) documents are not considered in deciding whether the claimed invention involves an inventive step. EPC art. 56. The EPO rule is therefore narrower than the U.S. rule in substantive effect. A 54(3) document may destroy novelty if it directly and unambiguously discloses the claimed subject matter, but it cannot be combined with other references or used as an inventive-step starting point.

The Boards of Appeal have reinforced the strict novelty-only character of Article 54(3). In T 167/84, Fuel Injector Valve/Nissan, the Technical Board held that the “whole contents” of an earlier Article 54(3) application do not extend to undisclosed equivalents. T 167/84, Fuel Injector Valve/Nissan, 1987 O.J. E.P.O. 369. That conclusion follows from the structure of the EPC: because Article 54(3) references are excluded from inventive step, their effect should not be expanded by equivalence or obviousness reasoning. Id.

The relevant “content” is the earlier European application as filed. EPO Guidelines for Examination G-IV, 5.1 (2026). The Guidelines state that the content includes the description, drawings, and claims, but does not include the priority document itself or the abstract. Id. The priority document matters only to determine whether the earlier application is entitled to the asserted priority date for the relevant disclosure.

VI.       Priority as the Central EPO Battleground

Because Article 54(3) turns on filing and priority dates, priority is often dispositive. The leading authority is G 2/98, where the Enlarged Board held that priority for the “same invention” is acknowledged only if the skilled person can derive the claimed subject matter directly and unambiguously, using common general knowledge, from the priority application as a whole. G 2/98, Requirement for Claiming Priority of the “Same Invention,” 2001 O.J. E.P.O. 413.

That standard operates on both sides of an Article 54(3) dispute. The later application may need priority to predate the earlier-published reference. The earlier reference may need priority to qualify as an earlier filing against the later claim. If priority fails for one but not the other, the result can be asymmetric and outcome-determinative.

G 1/15 is important because it limits some self-collision risks. In that decision, the Enlarged Board held that partial priority may not be refused for a generic “OR” claim, provided the relevant alternative subject matter was disclosed directly or implicitly, unambiguously, and in an enabling manner in the priority document. G 1/15, Partial Priority (Enlarged Bd. App. Nov. 29, 2016). Partial priority can therefore prevent an applicant’s own earlier filing or priority-family publication from becoming fatal against subject matter that is properly entitled to priority.

Priority entitlement itself has also been clarified. In G 1/22 and G 2/22, the Enlarged Board held that the EPO is competent to assess whether a party is entitled to priority, but that there is a rebuttable presumption that the applicant claiming priority is entitled to do so. G 1/22 & G 2/22, Entitlement to Priority, 2024 O.J. E.P.O. A50. Those decisions are particularly important in PCT-to-EP filing chains where the priority applicant and later applicant are not identical.

VII.   Euro-PCT Applications: Similar Fact Pattern, Different Gatekeeping

PCT filings create an additional divergence. In the United States, current AIA practice treats the WIPO publication of a PCT application designating the United States as a U.S. patent application publication for § 102(a)(2) purposes, regardless of international filing date, publication language, or U.S. national-stage entry. 35 U.S.C. §§ 102(a)(2), 374; MPEP § 2154.01(a).

The EPO rule is more conditional. A PCT application designating Europe can become Article 54(3) prior art only if the EPC conditions are met. Rule 165 EPC, read with Article 153(5), provides the framework. EPC art. 153(5); EPC r. 165. The 2026 EPO Guidelines state that an international application is included in the Article 54(3) state of the art if the PCT applicant has paid the filing fee under Rule 159(1)(c) and supplied a translation under Rule 159(1)(a) where the application was not published by the International Bureau in an EPO official language. EPO Guidelines for Examination A-XIII, 9.1.3 (2026). The Guidelines further state that not all minimum requirements for European-phase entry must be met for the Euro-PCT application to be treated as a conflicting European application. Id.

The practice point is straightforward. A U.S. practitioner should not assume that Euro-PCT Article 54(3) status follows automatically from WIPO publication. A European practitioner should not assume that U.S. § 102(a)(2) requires U.S. national-stage entry.

VIII.                  EPO Disclaimers and Same-Owner Collisions

Because the EPC has no common-ownership exception for Article 54(3), same-owner collisions must be solved through priority, claim drafting, novelty amendments, or disclaimers. Disclaimers are therefore not merely a prosecution tool; they expose the EPC’s refusal to solve same-owner Article 54(3) problems through ownership rules.

The Enlarged Board’s disclaimer cases allow only a constrained response. In G 1/03, the Enlarged Board held that an undisclosed disclaimer may be allowable to restore novelty over Article 54(3) prior art, but only if the disclaimer removes no more than necessary and does not provide a technical contribution to the claimed subject matter. G 1/03, Disclaimer/PPG, 2004 O.J. E.P.O. 413. In G 1/16, the Enlarged Board reaffirmed the special G 1/03 framework for undisclosed disclaimers and emphasized that such disclaimers must satisfy strict added-matter limits. G 1/16, Disclaimer III, 2018 O.J. E.P.O. A70.

Recent Board decisions show the limits of this approach. In T 88/21, Glass Article/Corning, the Board addressed a same-applicant collision in which priority was not valid for the full claim scope and an undisclosed disclaimer based on the applicant’s own earlier disclosure was not allowed. T 88/21, Glass Article/Corning (Tech. Bd. App. Nov. 15, 2022). In T 1946/19, Memory Lock/CLEVX, the Board found lack of novelty where priority failed for the claim but was valid for the relevant subject matter of the Article 54(3) prior-art reference. T 1946/19, Memory Lock/CLEVX (Tech. Bd. App. Nov. 9, 2023). Those decisions illustrate that Article 54(3) often turns less on ownership than on the granularity of priority support.

IX.       Strategic Consequences for Global Portfolios

The U.S. rule is more dangerous in one respect and more forgiving in another. It is more dangerous because a § 102(a)(2) reference may be used for obviousness under § 103. See 35 U.S.C. §§ 102(a)(2), 103; Hazeltine, 382 U.S. at 254–56. But it is more forgiving because § 102(b)(2) may remove inventor-origin disclosures, prior inventor public disclosures, and commonly owned disclosures from the § 102(a)(2) prior-art base. 35 U.S.C. § 102(b)(2).

The EPO rule is narrower because Article 54(3) is novelty-only. EPC art. 56. But it can be harsher in same-owner portfolio practice because the EPC has no equivalent to § 102(b)(2)(C). The same corporate family’s earlier EP application can be a novelty-destroying Article 54(3) reference against a later EP application if the statutory requirements are met and priority does not save the claim.

For U.S. prosecution, the response checklist should be: identify whether the reference is a qualifying U.S. patent document; determine the subject-matter-specific effective filing date under § 102(d); test the “names another inventor” requirement; evaluate § 102(b)(2) exceptions; and consider double-patenting separately. See 35 U.S.C. §§ 102(a)(2), 102(b)(2), 102(d), 103; MPEP §§ 2154.01, 717.02(a).

For EPO prosecution, the response checklist should be different: confirm that the reference is an EP application or qualifying Euro-PCT application; verify publication timing; test priority for both the claim and the reference; ask whether the earlier application directly and unambiguously discloses the claimed subject matter; and consider a targeted amendment or permissible disclaimer. See EPC arts. 54(3), 56, 89; EPO Guidelines G-IV, 5.1; EPO Guidelines A-XIII, 9.1.3.

X.          Conclusion

Earlier-filed, later-published patent applications occupy an unusual place in patent law. They were not public when the later applicant filed, but they may acquire prior-art effect once they publish. The U.S. and European systems share that basic idea, but they implement it differently.

The U.S. system treats secret patent filings as part of the general prior-art universe, then mitigates harshness through statutory exceptions. The EPC takes the opposite approach: it sharply limits secret filings to novelty, but offers no ownership-based escape from self-collision. That policy divergence explains why the same filing chronology can produce different prosecution strategies in the USPTO and EPO.

In the United States, current AIA analysis begins with §§ 102(a)(2) and 102(d), not with pre-AIA case law. The pre-AIA authorities remain useful, but their doctrinal role is limited and context-specific. Milburn supplies the historical justification for secret prior art. Hazeltine supplies the historical obviousness principle. Dynamic Drinkware, Riggs, and Lynk Labs illuminate pre-AIA priority and publication-date issues, but the AIA inquiry starts with the statutory text and current USPTO guidance.

At the EPO, Article 54(3) is a novelty-only rule. Article 56 excludes Article 54(3) documents from inventive step, and cases such as T 167/84 confirm that the doctrine should not be expanded through equivalents or obviousness reasoning. But the EPC’s narrower effect comes with fewer escape routes: there is no U.S.-style common-ownership exception. Priority, direct-and-unambiguous disclosure, and carefully limited disclaimers do the work.

For global filing strategy, the core lesson is simple. In the United States, ask whether the reference qualifies, what its subject-matter-specific effective filing date is, whether it names another inventor, and whether an AIA exception removes it. In Europe, ask whether the earlier EP or Euro-PCT application directly and unambiguously discloses the claim, whether priority changes the answer, and whether the claim can be amended or disclaimed without adding matter.