OF 99 MENTALLY RETARDED MALE OFFENDERS IN 3 INSTITUTIONS, 7 HAD ABNORMAL CHROMOSOME CONFIGURATIONS--47,XYY IN 6 CASES, 47,XXY IN 1, AND 48,XXYY IN 1. THE PREVALENCE OF XYY INDIVIDUALS (6.5 PERCENT) IS 50 TIMES THAT OF THE GENERAL POPULATION. THE PERSONALITIES OF THE CHROMOSOMALLY ABNORMAL PATIENTS WERE ANALYZED THROUGH OBSERVATION AND PSYCHOLOGICAL TESTING. THEIR CRIMINAL RECORDS, IQ'S (INTELLIGENCE QUOTIENTS), AND HEIGHT WERE COMPARED WITH THOSE OF THE 91 MENTALLY RETARDED OFFENDERS WITH NORMAL CHROMOSOME PATTERNS. THE CHROMOSOMALLY ABNORMAL OFFENDERS HAD HIGHER IQ'S (63-66) THAN OTHER RETARDED OFFENDERS, SCORED BETTER ON TESTS OF PERCEPTUAL ORGANIZATION THAN ON TESTS OF VERBAL COMPREHENSION AND ATTENTION, HAD VERY SHORT ATTENTION SPANS, AND WERE UNUSUALLY TALL. IN ADDITION, THEIR PERSONALITY TESTS INDICATED THAT THEY WERE IMPULSIVE, IMMATURE, INCLINED TO USE PRIMITIVE AND INEFFECTIVE DEFENSE MECHANISMS, AND EASILY FRUSTRATED. THE STUDY ALSO INCLUDED A CHROMOSOME ANALYSIS OF 30 MENTALLY RETARDED SCHOOL BOYS WHO HAD BEEN INVOLVED IN PETTY LARCENY AND HAD DEMONSTRATED AGGRESSIVENESS. ONE BOY WITH A 47,XXY CONFIGURATION WAS FOUND. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED.