On May 25, 2011, the U.S. Supreme Court decided the case of Therasense, Inc., et al. v. Becton, Dickinson and Company, et al., which changed the standards for a finding of inequitable conduct which is an equitable defense to patent infringement. If proven, inequitable conduct bars enforcement of a patent. Prior to Therasense, the standards for a finding of inequitable conduct involved a broader scope of misconduct, including not only intentional acts of misconduct intended to deceive the USPTO and the courts, but also merely not disclosing information to the USPTO with no intent to deceive. Because of this broader scope and the strong remedy of unenforceability, a finding of inequitable conduct came to require a finding of materiality. The standards for intent to deceive and materiality have fluctuated over time. In the past, the intent requirement was satisfied based on gross negligence or even negligence. The court had also previously adopted a broad view of materiality using a “reasonable examiner” standard (a reference is material if there is a substantial likelihood that a reasonable examiner would consider it important in deciding whether to allow an application to issue as a patent). Further weakening the showing needed to establish inequitable conduct, the court then placed intent and materiality on a sliding scale (holding patents unenforceable based on a reduced showing of intent if the record contained a strong showing of materiality and vice versa). The court embraced the reduced standards for materiality and intent to encourage full disclosure to the USPTO, which had numerous unintended consequences. Most prominently, inequitable conduct became a significant litigation strategy. Because the doctrine focused on the moral turpitude of the patentee with ruinous consequences for the reputation of his patent attorney, it discouraged settlement and deflected attention from the merits of validity and infringement issues. Inequitable conduct disputes also increased the complexity, duration and cost of patent infringement litigation. A finding of inequitable conduct regarding a single claim renders the entire patent unenforceable. Further, it can also render unenforceable other related patents and applications in the same technology family. With these far-reaching consequences, it is no surprise that charges of inequitable conduct have become a popular patent litigation strategy. Because of the broad view of intent and materiality, patent attorneys flooded the USPTO with a deluge of prior art references, a lot of which had minimal value, making it more difficult for examiners to identify relevant prior art. This resulted in a severe strain to the USPTO. In Therasense, the Court tightened the standards for intent and materiality in order to redirect a doctrine that was overused to the detriment of the public. The court found that to prevail on a claim of inequitable conduct, the accused infringer must prove that the patentee acted with specific intent to deceive the USPTO. A finding of negligence or gross negligence under a “should have known” standard does not satisfy the intent requirement. The accused infringer must prove that the patentee made a deliberate decision to deceive the USPTO. Further, the court found that intent and materiality are separate requirements and that a district court should not use a sliding scale where a weak showing of intent may be found sufficient based on a strong showing of materiality and vice versa. To meet the clear and convincing evidence standard, the specific intent to deceive must be the single most reasonable inference able to be drawn from the evidence. Previously, raising the intent standard alone did not reduce the number of inequitable conduct cases before the courts and did not cure the problem of overdisclosure to the USPTO. To address these concerns, the court in Therasense adjusted the standard for materiality. The court held that the materiality required to establish inequitable conduct is but-for materiality. When an applicant fails to disclose prior art to the USPTO, that prior art is but-for material if the USPTO would not have allowed a claim had it been aware of the undisclosed prior art. In assessing the materiality of a withheld reference, the court must determine if the USPTO would have allowed a claim had it been aware of the undisclosed material. As a major exception, the court here held that in cases involving affirmative egregious misconduct, but-for material need not be proven. Finally, the court held that courts should apply equity to ensure that the remedy is not imposed for misconduct that was immaterial to the issuance of the patent. Even when materiality and intent are proven, the court held that a patent should only be rendered unenforceable due to inequitable conduct “where the patentee’s misconduct resulted in the unfair benefit of receiving an unwarranted claim.” This equitable analysis is intended to serve as a substantial requirement that limits the discretion of district courts.