In a 7-1 ruling, the court said that when older workers are disproportionately affected by an employment decision, the employer bears the burden of explaining whether there was a reasonable explanation other than age for the company's action.

The case involves workers over 40 who challenged their dismissals from jobs at the Knolls Atomic Power Laboratory in upstate New York.

Thirty of the 31 workers laid off by the lab in 1996 were over 40. Twenty-six of those employees sued Knolls claiming that the layoffs violated the federal Age Discrimination in Employment Act.

Justice David Souter acknowledged, in his majority opinion, that the decision "makes it harder and costlier to defend" age discrimination lawsuits. But Souter said, "We have to read it the way Congress wrote it."