OBJECTIVE: The authors' goal was to determine whether changing the insanity test in California in 1982 actually affected use of the insanity defense. METHOD: They gathered information on all individuals who entered the insanity plea (N = 1,300) and all those acquitted by reason of insanity (N = 665) in seven counties in California 3 years before and 3 years after the change in the test of insanity. They focused on whether California's revision of the insanity test affected the number and rate of insanity pleas, the number and rate of insanity acquittals, the characteristics of individuals using the defense, the target offenses, and the length of stay of insanity acquittees. RESULTS: The reform did not affect the rate of insanity pleas or acquittals, the characteristics of those using the defense, or their length of confinement. CONCLUSIONS: Although the reform had no direct effects on the use of the insanity defense, the authors hypothesize that an indirect change in criminal sentencing practices led to a decrease in the number and rate of insanity pleas and acquittals.