THE STUDY SUBJECTS WHO HAD BEEN CONVICTED OF VIOLENT CRIMES OR OTHER ANTISOCIAL BEHAVIOR, RANGED IN AGE FROM 16 TO 61 YEARS (MEAN AGE 23 YEARS). HALF OF THE SAMPLE WAS SELECTED FOR HEIGHT (GREATER THAN 170 CM). SMEARS FROM BUCCAL MUCOSA WERE TESTED FOR X-CHROMATIN AND Y-CHROMATIN ANOMALIES WITH THIONIN AND QUINACRINE MUSTARD STAIN, RESPECTIVELY. NO CASE OF EITHER XXY OR XYY CHROMOSOMAL ANEUPLOIDY WAS DETECTED. FOUR INDIVIDUALS APPEARED TO HAVE THE XYY ANEUPLOIDY, BUT IT WAS FOUND THAT WHAT WERE THOUGHT TO BE SECOND Y-FLUORESCENT SPOTS WERE ACTUALLY FLUORESCENT SEGMENTS OF AUTOSOMES. A BRIEF DISCUSSION OF OTHER STUDIES OF CHROMOSOMAL ABNORMALITIES AMONG OFFENDERS CONCLUDES THAT MALES WITH THE XYY ANEUPLOIDY APPARENTLY SHOW A PSYCHOPATHOLOGY MARKED BY INCREASED IMPULSIVENESS, BUT THIS PSYCHOPATHOLOGY NEEDS FURTHER CHARACTERIZATION. TABLES OF SUPPORTING DATA AND 29 REFERENCES ARE INCLUDED.