Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co., Ltd.
United States Supreme Court
535 U.S. 722 (2002)
Festo (plaintiff) owned two patents on magnetic conveyor systems, and it had amended the claims during prosecution. Festo sued Shoketsu (defendant) for infringement under the doctrine of equivalents, which lets a patent holder reach devices that don't literally match the claim language but are functionally the same. Shoketsu argued that any narrowing amendment automatically forfeits equivalents coverage for that claim. The district court agreed with Shoketsu, the Federal Circuit affirmed en banc, and Festo appealed to the Supreme Court.
Whether an inventor who narrows a patent claim by amendment during prosecution is automatically barred from later invoking the doctrine of equivalents for that claim, regardless of the amendment's purpose.