THE PATIENTS WERE CONFINED TO HOSPITALS IN WISCONSIN, MICHIGAN, INDIANA, AND OHIO AND INCLUDED THOSE WHO WERE ACCUSED OF CRIME BUT CONSIDERED INCOMPETENT TO STAND TRIAL; OTHERS, WHOSE VIOLENT OR OTHERWISE ANTISOCIAL BEHAVIOR MADE THEM DIFFICULT MANAGEMENT PROBLEMS, WERE REFERRED FROM PENAL INSTITUTIONS AND HOSPITALS FOR THE MENTALLY RETARDED. THREE OF THE XYY PATIENTS WERE DIAGNOSED AS MENTALLY RETARDED, THREE AS SOCIOPATHIC, AND THREE AS SCHIZOPHRENIC. THE REMAINING PATIENT WAS DIFFICULT TO CLASSIFY BECAUSE OF SUGGESTIVE EVIDENCE OF PROGRESSIVE ORGANIC DISEASE OF THE CENTRAL NERVOUS SYSTEM. THE INCIDENCE OF HOMOSEXUALITY IN THESE PATIENTS WAS SIGNIFICANTLY HIGHER THAN IN CONTROLS FROM THE SAME POPULATION. NO PARTICULAR PROCLIVITY FOR OTHER TYPES OF CRIME WAS FOUND IN THE XYY PATIENTS. FIVE CAME FROM ENVIRONMENTS CONSIDERED DETRIMENTAL TO THE NORMAL DEVELOPMENT OF PERSONALITY AND CHARACTER. THE MEAN AGE AT FIRST ARREST IS LOWER FOR THE XYY'S THAN FOR THE CONTROLS, ALTHOUGH THE DIFFERENCE IS NOT SIGNIFICANT. IN SEVERAL RESPECTS, THESE FINDINGS DIFFER FROM OTHER STUDIES OF XYY MALES. FURTHER RESEARCH IS NECESSARY TO DETERMINE MORE PRECISELY WHAT ILL EFFECTS AN EXTRA Y CHROMOSOME MAY HAVE. REFERENCES ARE INCLUDED. (AUTHOR ABSTRACT MODIFIED)