The U.S. Supreme Court on May 28, 2002 in a unanimous decision vacated the Festo Corp. v. Shoketsu Kinzoku Kogyo Kabushiki Co. decision of the Court of Appeals for the Federal Circuit. The November 2000 decision held that (1) an amendment that narrows the scope of a claim for any reason related to the statutory requirements for a patent will give rise to prosecution history estoppel with respect to the amended claim element; (2) a "voluntary" claim amendment that narrows the scope of the claim for a reason related to the statutory requirements for a patent will give rise to prosecution history estoppel as to the amended claim element; (3) when a claim amendment creates prosecution history estoppel with respect to a claim element, there is no range of equivalents available for the amended claim element, thereby completely barring application of the doctrine of equivalents as to the amended element ("complete bar"); and (4) failure to explain a claim amendment invokes a presumption of prosecution history estoppel, thereby providing no range of equivalents under the doctrine of equivalents as to that element. The Court of Appeals decision was very controversial.

The Supreme Court acknowledged that the doctrine of equivalents introduces uncertainty into the issue of claim interpretation, however, the Court disagreed with the complete bar rule set out by Federal Circuit, preferring instead a flexible approach to the doctrine of equivalents. While the Court agreed that any narrowing amendment made for a reason related to patentability could give rise to prosecution history estoppel, the Court stated that the Federal Circuit ignored the instruction in Warner-Jenkinson "that courts must be cautious before adopting changes that disrupt the settled expectations of the inventing community. Inventors who amended their claims under the previous case law had no reason to believe that they were conceding all equivalents" of amended elements when responding to a rejection. Had they known, the Court stated, they might have appealed the rejection. The Court stated that, where claims are amended, "the inventor is deemed to concede that the patent does not extend as far as the original claim" and the patentee has the burden of showing that the amendment does not surrender the particular equivalent. The patentee must establish that the equivalent was unforeseeable at the time the claim was drafted, the amendment did not surrender the particular equivalent in question or there was some reason why the patentee could not have recited the equivalent in the claim.

A copy of the Supreme Court's decision, in pdf format, is available on-line.