Inequitable conduct
Inequitable conduct is an equitable defense that renders an entire patent — and in many circumstances related patents in the same family — unenforceable when the patentee, with specific intent to deceive, withheld or misrepresented information that was but-for material to the Patent and Trademark Office's allowance decision. Because its remedy is total, the Federal Circuit has called it the "atomic bomb" of patent law and, in Therasense, sharply tightened both prongs of the test.
The rule
A patent procured through inequitable conduct before the United States Patent and Trademark Office is unenforceable. To prevail, an accused infringer must prove, by clear and convincing evidence, that the patent applicant (1) made a misrepresentation or omission that was but-for material to the patent's issuance, and (2) acted with specific intent to deceive the PTO. Therasense, Inc. v. Becton, Dickinson & Co., 649 F.3d 1276, 1287, 1290 (Fed. Cir. 2011) (en banc).
The two prongs are separate. A district court may not infer intent solely from the materiality of the withheld reference, and may not weigh materiality and intent on a "sliding scale." Id. at 1290. Each must be proved on its own terms before the court conducts the equitable balancing that determines whether the patent should in fact be held unenforceable.
The remedy is severe. Unlike anticipation or obviousness, which invalidate only those claims that fail their respective tests, a finding of inequitable conduct renders the entire patent unenforceable, and the doctrine of "infectious unenforceability" can extend the bar to other patents in the same family that are tainted by the misconduct. See Consol. Aluminum Corp. v. Foseco Int'l Ltd., 910 F.2d 804 (Fed. Cir. 1990).
Statutory and constitutional source
Inequitable conduct is judge-made equity rather than statute. It descends from the Supreme Court's "unclean hands" trilogy: Keystone Driller Co. v. General Excavator Co., 290 U.S. 240 (1933); Hazel-Atlas Glass Co. v. Hartford-Empire Co., 322 U.S. 238 (1944); and Precision Instrument Manufacturing Co. v. Automotive Maintenance Machinery Co., 324 U.S. 806 (1945). Each refused to enforce a patent where the patentee had practiced fraud on a court or on the Patent Office.
The administrative scaffolding is found in 37 C.F.R. § 1.56, which imposes an affirmative duty of candor and good faith on each individual associated with the filing or prosecution of a patent application — including inventors, attorneys, and agents. The rule requires disclosure of all information known to be material to patentability. The PTO promulgated § 1.56 under its rulemaking authority in 35 U.S.C. § 2(b)(2), but a violation of the regulation is not, by itself, inequitable conduct: the Federal Circuit in Therasense made clear that the judicial standard is more demanding than the agency's disclosure standard.
Congress has spoken obliquely. The Leahy-Smith America Invents Act of 2011 created a "supplemental examination" procedure, 35 U.S.C. § 257, that allows a patent owner to ask the PTO to consider information that might otherwise support an inequitable-conduct defense; if the PTO determines no substantial new question of patentability is raised (or if it conducts and concludes a reexamination), the information cannot be the basis of a later inequitable-conduct holding, with limited exceptions. The supplemental examination cure is not retroactive to charges of fraud already pending in litigation. See 35 U.S.C. § 257(c).
The framework the courts apply
But-for materiality
Under Therasense, materiality is determined under a but-for standard: information is material if the PTO would not have allowed the claim had it been aware of the undisclosed prior art, applying a preponderance-of-the-evidence standard and giving claims their broadest reasonable construction. 649 F.3d at 1291. The court rejected as too lax the older Rule 56 "reasonable examiner" formulation that treated information as material whenever it was the kind of thing an examiner would want to consider.
Importantly, the court in Therasense recognized that a claim that is held invalid in litigation is not necessarily one whose allowance turned on the withheld reference; conversely, a claim that survives validity challenges may still have issued only because the examiner was kept in the dark. The materiality inquiry asks a counterfactual question about the prosecution record, not whether the claim is in fact valid.
Egregious misconduct exception
The court carved out a narrow exception for "affirmative egregious misconduct": filing a false affidavit, submitting fabricated test data, or like acts of deliberate fraud are deemed material per se without separate but-for analysis. 649 F.3d at 1292. Mere nondisclosure of prior art does not qualify, however serious; the egregious-misconduct exception is reserved for affirmative deception.
Specific intent to deceive
The accused infringer must prove that the applicant knew of the reference, knew that it was material, and made a deliberate decision to withhold it. Therasense, 649 F.3d at 1290. Gross negligence, even gross negligence about a known reference, is not enough. The intent inference must be the "single most reasonable inference able to be drawn from the evidence." Id. (quoting Star Scientific, Inc. v. R.J. Reynolds Tobacco Co., 537 F.3d 1357, 1366 (Fed. Cir. 2008)).
Direct evidence of intent is rare. Courts therefore rely on circumstantial evidence: the timing of the withholding, the prosecutor's knowledge of the reference, the obviousness of its relevance, and the absence of any credible good-faith explanation. The applicant has no obligation to come forward with a good-faith explanation, but in close cases the absence of any may tip the inference toward intent.
Equitable balancing
If both prongs are proved by clear and convincing evidence, the court still must balance the equities. Therasense, 649 F.3d at 1287. In practice, where but-for materiality and specific intent are both shown, courts almost invariably find the patent unenforceable.
Burden and proof
Inequitable conduct must be pleaded with the particularity required by Federal Rule of Civil Procedure 9(b). The Federal Circuit's decision in Exergen Corp. v. Wal-Mart Stores, Inc., 575 F.3d 1312 (Fed. Cir. 2009), requires that the pleading identify the specific who, what, when, where, and how of the misconduct, and recite facts from which a court may reasonably infer that an individual associated with the prosecution both knew of the withheld information and knew of its materiality. Conclusory allegations of "fraud on the Patent Office" do not survive a Rule 12(b)(6) motion.
At trial, the burden is clear and convincing evidence on both prongs. Therasense, 649 F.3d at 1287. This is the same elevated standard that governs anticipation and obviousness challenges and reflects the statutory presumption of validity in 35 U.S.C. § 282 — a presumption that, while doctrinally separate from enforceability, is conventionally applied to inequitable-conduct claims as well.
Inequitable conduct is tried to the court rather than to the jury. The court makes underlying findings of fact reviewed on appeal for clear error, but the ultimate decision to hold a patent unenforceable is reviewed for abuse of discretion. Id. at 1290–91.
Interaction with related doctrines
Inequitable conduct sits at the boundary of validity, enforceability, and litigation misconduct. Its most direct neighbor is prosecution history estoppel: both turn on a careful reading of what was said and not said during prosecution, but estoppel narrows claim scope under the doctrine of equivalents, while inequitable conduct kills the patent.
Where the same misconduct is pleaded as a Walker Process antitrust counterclaim, the standard converges with common-law fraud rather than the somewhat looser Therasense showing; Walker Process Equipment, Inc. v. Food Machinery & Chemical Corp., 382 U.S. 172 (1965), requires proof that the patent was procured by knowing and willful fraud. The Federal Circuit has explained that post-Therasense, the two standards now overlap substantially. See TransWeb, LLC v. 3M Innovative Properties Co., 812 F.3d 1295 (Fed. Cir. 2016).
Inequitable conduct also frequently interacts with the exceptional-case attorney-fee inquiry under 35 U.S.C. § 285. A finding of inequitable conduct, while not automatically requiring a fee award, ordinarily supports one. Octane Fitness, LLC v. ICON Health & Fitness, Inc., 572 U.S. 545 (2014), governs the underlying fee analysis. See Octane Fitness v. ICON.
The defense may be raised in the district court but cannot be litigated in inter partes review; PTAB proceedings are limited to anticipation and obviousness over patents and printed publications. 35 U.S.C. § 311(b). Conversely, supplemental examination under § 257 offers a partial cure available only at the PTO.
Practical notes
For prosecutors, the post-Therasense regime has shifted day-to-day practice. The Federal Circuit warned that the older "reasonable examiner" materiality test had bred an "absurd" practice of burying examiners in marginal references to ward off later inequitable-conduct allegations. 649 F.3d at 1289. The but-for standard is meant to recalibrate disclosure toward genuine materiality. In practice, however, careful counsel continue to err on the side of disclosure: the cost of an Information Disclosure Statement entry is low, the cost of an unenforceability finding is total.
For litigators, Exergen imposes a real pleading hurdle. Stock allegations recycled from one defendant's answer to the next will not survive a motion to dismiss. The pleading must identify a specific reference, a specific individual who knew of it, a specific reason that individual knew the reference was material, and a specific deceptive act or omission. Discovery directed at prosecution counsel's files — including drafts, internal memoranda, and parallel foreign-prosecution disclosures — is the usual way to develop the record.
Three areas continue to generate litigation. First, prior-art disclosures made to one examiner during prosecution of a parent application but omitted from a continuation: omission is rarely material because the second examiner has access to the parent file, and the Federal Circuit has generally found no inequitable conduct on these facts. See, e.g., 1st Media, LLC v. Electronic Arts, Inc., 694 F.3d 1367 (Fed. Cir. 2012). Second, foreign search reports and office actions citing references not disclosed in the U.S. application: these can be material under Therasense if they would have changed the U.S. examiner's allowance decision. Third, undisclosed clinical or test data underlying patentability arguments — the closest analogue to the affirmative-misrepresentation egregious-misconduct exception.
Counsel should also preserve the option of supplemental examination under § 257 when material information is discovered post-issuance but before or during litigation. The window for invoking supplemental examination closes once an inequitable-conduct allegation has been pleaded with particularity in court, but if invoked in time, it can defuse the defense entirely. 35 U.S.C. § 257(c).
Open questions
Several issues remain unsettled after Therasense. The "egregious misconduct" exception has rarely been applied; lower courts have struggled to articulate when an affirmative misstatement, as opposed to a nondisclosure, crosses the line from advocacy into fraud. The treatment of false or misleading attorney argument — characterizations of prior art that an examiner relies upon — is in particular flux.
A second open area is the scope of "infectious unenforceability" after Therasense. The doctrine, originally articulated under the more permissive pre-Therasense framework, has been applied unevenly to continuation, divisional, and continuation-in-part patents. Whether a single act of inequitable conduct in a parent prosecution renders unenforceable claims that issued only after the misrepresentation was effectively cured remains a fact-bound question without clear resolution.
Finally, the relationship between supplemental examination and litigation remains under-developed. Whether a successful supplemental examination forecloses every flavor of inequitable-conduct theory — including egregious affirmative misconduct — or only those tied to specific items of withheld information, has not been fully tested.
See also
- AnticipationThe §102 invalidity analysis whose underlying references most often supply the "but-for material" art in inequitable-conduct disputes.
- ObviousnessA withheld reference is most often material because it would have rendered the claim obvious under §103.
- Prosecution history estoppelA doctrine that, like inequitable conduct, turns on a close reading of what was said and done during prosecution.
- Exceptional caseFindings of inequitable conduct frequently support fee awards under 35 U.S.C. §285.
- Octane Fitness v. ICONGoverns the discretion under which fees may follow an inequitable-conduct finding.
- Inter partes reviewA PTAB proceeding that, unlike district-court litigation, cannot adjudicate inequitable conduct.
- GlossaryDefinitions of "materiality," "specific intent," and related terms.
Last reviewed: 2026