The US Supreme Court has affirmed that an issued patent has a presumption of validity that requires a high standard of proof to overcome. In Microsoft Corp. v. i4i Limited Partnership,[1] the Supreme Court affirmed that an invalidity defense in a patent case must be proven by clear and convincing evidence, rather than the simple preponderance of the evidence applied to other patent issues, such as the question of whether infringement has occurred. The Supreme Court rejected the argument by Microsoft that the patent laws do not mandate this elevated level of proof for invalidity. That argument was rejected even in those instances in which the patent challenger relies on pre-existing technology that the Patent Office was unaware of and did not consider while issuing the patent. This ruling helps ease the concerns of patent owners that the strength of US patents would soon be eroded.

During the application process the Patent Office determines whether to issue a patent by comparing the claimed invention to pre-existing technology, referred to as prior art. During a subsequent court proceeding, such as one in which the owner charges infringement, a defense commonly raised is the invalidity of the patent. Often that defense is based upon prior art that had not been cited by the Patent Office in its original patentability determination. The patent statute, both before and after the changes enacted in September 2011, includes an explicit provision that an issued patent is presumed to be valid and the burden of establishing the invalidity of a patent rests on the party challenging validity.

Notwithstanding this presumption of validity extended by the patent statute, Microsoft argued that the burden of proving validity did not require a heightened standard of proof. Microsoft argued that at a minimum a heighted standard should not apply in those instances in which the challenger was relying on prior art that the Patent Office had not considered during the original application process. There was precedent to support such an argument. Prior to the formation of the Court of Appeals for the Federal Circuit in 1982, a number of court decisions stated that the presumption of validity could be lessened, if not destroyed, by the citation of prior art that had not been considered by the Patent Office. Shortly after its formation, the Court of Appeals for the Federal Circuit embarked on a long series of decisions establishing that the presumption of validity created a heightened standard for an invalidity defense regardless of whether newly discovered prior art was relied upon. The Supreme Court has not always agreed, however, with tests and standards established by the Federal Circuit Court of Appeals.

In 2007, the Supreme Court reversed another test that had been established by the Court of Appeals for the Federal Circuit for determining whether a patent was invalid as obvious over the prior art. Over a period of years the Court of Appeals for the Federal Circuit had established a test for obviousness of a patent by which a challenger was required to prove a teaching, suggestion or motivation in the prior art to combine or modify the prior art. In KSR International Co. v. Teleflex Inc.[2] the Supreme Court rejected this teaching, suggestion or motivation test, and many practitioners perceived this decision as making easier the task of proving a patent invalid. When it was announced that the Supreme Court would consider the Microsoft v. i4i ruling and the issue of standard of proof for invalidity, many practitioners were concerned that the ruling might reflect a trend along the lines of the KSR decision.

Those concerns were misplaced. In the Microsoft decision, the Supreme Court analyzed the history of the presumption of validity that was enacted by Congress and the resulting increased standard of proof for invalidity. Finding the standard to be consistent with the history of the patent law, the Supreme Court made a sweeping determination that the clear and convincing evidence standard applies in patent invalidity cases. The Supreme Court recognized that in cases in which newly discovered prior art is relied upon the standard may be easier to meet, but the clear and convincing evidence standard still applies.