535 U.S. 722 (2002), Supreme Court of the United States
Festo is a foundational Supreme Court decision that recalibrates the balance between two core features of U.S. patent law: the notice function of patent claims and the equitable, gap-filling doctrine of equivalents (DOE).
Does a narrowing claim amendment made during prosecution for reasons related to patentability create a complete bar to asserting the doctrine of equivalents for the amended element, and do such estoppel effects also arise from amendments made to satisfy statutory requirements other than novelty and nonobviousness?
Prosecution history estoppel arises when a patentee narrows a claim to comply with any statutory requirement of the Patent Act, including but not limited to novelty, nonobviousness, and definiteness. Such a narrowing amendment gives rise to a presumption that the patentee has surrendered all equivalents between the original broader claim and the amended claim for the particular element amended. The presumption is rebuttable. A patentee may overcome it by showing that at the time of the amendment (1) the alleged equivalent was unforeseeable, (2) the rationale underlying the amendment bears only a tangential relation to the equivalent in question, or (3) there is some other reason suggesting the patentee could not reasonably have been expected to describe the alleged equivalent. The burden to rebut the presumption rests on the patentee.
The Supreme Court rejected the Federal Circuit's complete bar. A narrowing amendment related to patentability triggers a rebuttable presumption that the patentee surrendered all equivalents for the amended element, but the patentee may overcome that presumption by showing unforeseeability, tangential relation, or other justifying reasons. Prosecution history estoppel applies to amendments made to satisfy any patentability-related requirement, not only to avoid prior art. The judgment was vacated and remanded for application of this framework.
Festo is the modern anchor for prosecution history estoppel. It replaced the Federal Circuit's bright-line complete bar with a structured, rebuttable presumption that meaningfully restricts, but does not abolish, the doctrine of equivalents after narrowing amendments. For law students, Festo is essential for understanding claim drafting strategy, file history management, litigation burdens, and how courts weigh foreseeability and the tangentiality of amendments. The decision also confirms that § 112-driven amendments can produce estoppel, making precision in specification and claim language critical. On remand, the Federal Circuit applied Festo's test and found no equivalents for the "magnetizable sleeve" limitation but allowed potential equivalents for the "pair of sealing rings" limitation, illustrating the doctrine's element-by-element, fact-intensive application.