SPECIFICALLY, THE SAMPLE COMPRISED 61 PATIENTS WITH KLINEFELTER'S SYNDROME AND MORE THAN ONE X CHROMOSOME IN ALL OR PART OF THEIR CELLS, AND 12 PATIENTS WITH XYY (EXTRA Y CHROMOSOME) SYNDROME. DIAGNOSES WERE MADE BY SEX CHROMATIN EXAMINATIONS ON FEULGEN STAINED BUCCAL SMEARS AND CHROMOSOME ANALYSES ON LEUCOCYTE CULTURES. CRIMINALITY WAS REGISTERED ONLY IF THE PATIENTS HAD BEEN CHARGED FOR CRIMINAL ACTS, AND THE AGE AT FIRST CRIMINALITY IS THE AGE AT THE FIRST CHARGE. INFORMATION WAS ACCUMULATED ON THE PATIENTS' INTELLIGENCE QUOTIENTS, MENTAL ILLNESS HISTORIES, AND ENVIRONMENT DURING CHILDHOOD; ELECTROENCEPHALOGRAPHY (EEG) TESTS WERE PERFORMED ON PATIENTS WHO WERE REGISTERED CRIMINALS. SCORES FOR PATIENTS WITH THE XYY SYNDROME AND KLINEFELTER'S SYNDROME WERE AVERAGED AND COMPARED. RESULTS SHOW THAT THE MEAN AGE AT FIRST CRIMINALITY IS SIGNIFICANTLY LOWER IN PATIENTS WITH THE XYY SYNDROME (18.5 YEARS) THAN IN PATIENTS WITH KLINEFELTER'S SYNDROME (24.1 YEARS). THE DISPOSITION TO CHARACTER DEVIATION AND CRIMINALITY IS MORE PRONOUNCED IN PATIENTS WITH THE XYY SNDROME, AND THE SUBJECTS EVIDENCE COMPARATIVELY HIGHER FREQUENCIES OF VIOLENCE. A HIGH RATE OF SEXUAL CRIMINALITY WAS NOTICED IN PATIENTS WITH KLINEFELTER'S SYNDROME AS WELL AS THOSE WITH THE XYY SYNDROME. THE FINDING OF A CRIMINALITY PATTERN IN PATIENTS WITH KLINEFELTER'S AND THE XYY SYNDROME IS SIMILAR TO WHAT IS FOUND IN PATIENTS WITH MENTAL RETARDATION, INDICATING THAT LOW INTELLIGENCE MAY HAVE BEEN A FACTOR IN CRIMINALITY. THE EEG STUDIES DO NOT INDICATE ANY CORRELATION BETWEEN EEG ABNORMALITIES AND A COMPARATIVELY HIGH RISK OF CRIMINALITY IN PATIENTS WITH KLINEFELTER'S AND THE XYY SYNDROME. TABULAR DATA AND REFERENCES ARE PROVIDED. (DAG)