SUBJECTS IN THE STUDY WERE REFERRED TO THE FORENSIC UNIT OF THE INSTITUTE FROM THE COOK COUNTY CRIMINAL COURT OF ILLINOIS BETWEEN 1974 AND 1976. THEY HAD BEEN INDICTED ON VARIOUS FELONY CHARGES AND FOUND UNFIT TO STAND TRIAL. THE FORMAL PSYCHIATRIC DIAGNOSIS OF THE PATIENTS, RELEVANT DEMOGRAPHIC DATA, AND THEIR INDEX CRIMES WERE RECORDED. WHEN POSSIBLE, RECORDS OF PREVIOUS PSYCHIATRIC HOSPITALIZATIONS WERE OBTAINED. MOST PATIENTS WERE YOUNG, BLACK, OF LOW EDUCATION, AND UNMARRIED, WITH A HISTORY OF PREVIOUS ARRESTS AND CONVICTIONS. ABOUT HALF HAD PREVIOUS PSYCHIATRIC HOSPITALIZATIONS. INQUIRY INTO THEIR FAMILY HISTORIES SHOWED THAT MOST CAME FROM BROKEN HOMES AND LIVED IN CONDITIONS OF POVERTY. RESOURCES TO VIOLENCE AS A MEANS OF OBTAINING MONEY OR SOLVING INTERPERSONAL CONFLICT WAS AN EXPECTED NORM BY AT LEAST A PORTION OF THEIR SOCIAL MILIEU. THE MOST COMMON PSYCHIATRIC DIAGNOSIS WAS SCHIZOPHRENIA, FOLLOWED BY PERSONALITY DISORDERS, MENTAL RETARDATION, ORGANIC BRAIN SYNDROME, AND MANIC-DEPRESSIVE PSYCHOSIS. THE LEADING OFFENSE WAS ARMED ROBBERY, FOLLOWED BY HOMICIDE, BURGLARY, RAPE, THEFT, AGGRAVATED BATTERY, ATTEMPTED HOMICIDE, UNLAWFUL USE OF A WEAPON, ATTEMPTED RAPE, ARSON, INDECENT LIBERTIES WITH A MINOR, AND CRIMINAL DESTRUCTION OF PROPERTY. SCHIZOPHRENIA WAS THE LEADING PSYCHIATRIC DIAGNOSIS AMONG THE TOTAL POPULATION, AS WELL AS AMONG ACCUSED PERSONS FOR ALL TYPES OF OFFENSES EXCEPT ARSON. ORGANIC BRAIN SYNDROME WAS ASSOCIATED EXCLUSIVELY WITH VIOLENT CRIMES, WHEREAS MENTAL RETARDATION WAS FOUND PRIMARILY AMONG PERSONS ACCUSED OF MINOR CRIMES. SUPPORTING DATA ARE TABULATED, AND REFERENCES ARE CITED. (DEP)