CHROMOSOME ANALYSES WERE PERFORMED FOR ALL 42 FORENSIC PATIENTS IN THE CLINIC ON A GIVEN DAY AND FOR ALL 169 FORENSIC PATIENTS ADMITTED TO THE CLINIC OVER A 2.5-YEAR PERIOD. ALL TYPES OF CHROMOSOME ABERRATIONS WERE ASSESSED, WITH PARTICULAR ATTENTION TO THE SIZE OF THE Y CHROMOSOME. THE FREQUENCY OF KLINEFELTER'S SYNDROME (THE 47,XXY CHROMOSOME CONFIGURATION) WAS 0.5 PERCENT. THE 1.9 PERCENT FREQUENCY FOR THE 47,XYY SYNDROME IS SIGNIFICANTLY HIGHER THAN THAT (0.15 PERCENT) FOUND IN THE GENERAL POPULATION. THE FREQUENCY OF ENLARGED Y CHROMOSOME WAS 8.1 PERCENT, COMPARED TO 1.5 AND 1.4 PERCENT FOUND IN GENERAL POPULATION SAMPLES. THE FREQUENCY OF OTHER MARKER (UNUSUALLY LARGE) CHROMOSOMES WAS NOT GREATER THAN EXPECTED. THE MEAN Y/F INDEX (RATIO OF THE LENGTH OF THE Y CHROMOSOME TO THAT OF THE F CHROMOSOME) WAS 0.91, COMPARED WITH 0.83 IN A GENERAL POPULATION SAMPLE. TO TEST THE RELATIONSHIP OF THE Y CHROMOSOME SIZE TO TYPE OF CRIMINALITY AND MENTAL ILLNESS, PATIENTS WITH 46 CHROMOSOMES (THE NORMAL CONFIGURATION) WERE DIVIDED INTO TWO GROUPS: THOSE WITH RELATIVELY HIGH Y/F RATIOS (LARGE Y CHROMOSOMES), AND THOSE WITH LOWER RATIOS. PATIENTS WITH HIGHER Y/F RATIOS WERE MORE LIKELY TO HAVE COMMITTED VIOLENT CRIMES AND TO HAVE BEEN ADMITTED TO THE PSYCHIATRIC CLINIC, ALTHOUGH THERE WERE NO DIFFERENCES IN DIAGNOSTIC DISTRIBUTION BETWEEN THE TWO GROUPS. ANALYSIS OF THE FINDINGS IN LIGHT OF OTHER RESEARCH INDICATES THAT AN INCREASED RISK OF PERSONALITY DEVIATION AND OF VIOLENT CRIMINALITY IS ASSOCIATED WITH BOTH THE DOUBLE Y CHROMOSOME CONFIGURATION AND ENLARGEMENT OF THE Y CHROMOSOME. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)