Data were obtained from individual records from the departments of mental health, correction, and probation. Information was obtained on 166 persons who were found not guilty by reason of insanity of criminal charges, on 522 persons found guilty of 1 of 4 major offenses who were sentenced to a State correctional facility, and on 203 persons found guilty of 1 of these some 4 offenses who were sent to a county correctional facility. Most NGI offenders (91.6 percent) and white (81.9) percent, with a mean age of 32.8; in contrast, 69.8 percent of the State prisoners were white. Data suggest that the NGI population is older and more likely to be white than the prison population. Further, murderers represent a very small proportion (4.8 percent) of the NGI population; more than half (57.1 percent) of NGI offenders committed crimes against persons, including assault and battery, rape, robbery, and kidnapping. Less than 1 percent of all persons who were determined to have committed a crime and were subsequently institutionalized were found not guilty by reason of insanity. NGI offenders with serious disorders such as schizophrenia, paranoia, and organic brain syndrome were hospitalized an average of 309 days, compared with an average of 200 days for those with less serious disorders such as neuroses. NGI's were detained for significantly shorter lengths of stay than those sent to State prisons, but for longer periods than those sent to county correctional facilities. Data do not support the contention that the insanity defense is used successfully to avoid incarceration for murder or other serious offenses. Thirteen references and 18 tables are included.