Public opinion data reveal that the most prevalent concern expressed regarding the insanity defense is that it is a loophole through which would-be criminals escape punishment for illegal acts. This study used data from a longitudinal study of insanity defense reform in California, Georgia, Montana, New jersey, New York, Ohio, Washington, and Wisconsin. Overall, 976,209 felony indictments from 49 counties were examined to obtain data on all 8,953 criminal defendants who entered an insanity plea at any time during their defense between 1976 and 1985. The analysis focused on the use, success, and outcomes associated with the insanity defense. Results revealed that the public overestimates the use and success of an insanity defense and underestimates the extent to which individuals acquitted for reasons of insanity are confined upon acquittal. The authors concluded that selective media reporting is largely responsible for inaccurate perceptions of the insanity defense, but recent positive changes have occurred in media reports. Recognizing that public opinion is often the impetus for legal change, unquestioning acceptance of public opinion in the formation of laws and legal reform must be avoided until inaccurate public perceptions are changed. Tables and 19 references (Author abstract modified)