SUBJECTS OF THE NINE CASE HISTORIES CONSIST OF THREE MALES IDENTIFIED AS XYY'S IN PREVALENCE STUDIES AND SIX IDENTIFIED IN ROUTINE CLINICAL EXAMINATIONS. MOST OF THE MALES HAVE A NORMAL MASCULINE PHYSIQUE COMBINED WITH A TALL STATURE. THEY DEMONSTRATE WIDE VARIATIONS IN ABILITIES AND BEHAVIOR, ALTHOUGH THE THREE SUBJECTS LOCATED IN CHROMOSOME SURVEYS ARE MUCH MORE DEVIANT THAN THEIR COUNTERPARTS. NOTABLE STUDIES ON THE XYY SYNDROME AND FACTORS IN AGGRESSION HAVE PRODUCED A VARIETY OF CONCLUSIONS, SOME RESULTING FROM FINDINGS AS UNSUBSTANTIAL AS THOSE OBTAINED IN THE CASE HISTORIES. MALES WITH XYY CHROMOSOMES ARE SLIGHTLY MORE LIKELY THAN XY MALES TO BE PLACED EITHER IN CRIMINAL OR MENTAL INSTITUTIONS, AND ARE GENERALLY TALLER THAN XY MALES. THEY ARE USUALLY ADMITTED TO A MENTAL-PENAL INSTITUTION BECAUSE OF SOCIAL DEVIANCE RATHER THAN FOR CONVICTION OF A CRIMINAL OFFENSE (THEIR BEHAVIORAL CHARACTERISTICS INCLUDE TEMPER OUTBURSTS, SUDDEN FITS OF VIOLENT RAGE, AND AGGRESSIVENESS). UNUSUAL CHROMOSOME COUNTS IN AN INDIVIDUAL MAKE FOR A HIGHER RISK OF DEVIANT BEHAVIOR THAN ANY SPECIFIC DIFFERENCES BETWEEN THE 'Y' AND 'X' CHROMOSOME, AND NO SIGNIFICANT CORRELATION CAN BE ESTABLISHED BETWEEN MALE HORMONES AND AGGRESSION. STUDIES ON AGGRESSION AND BRAIN ABNORMALITIES SUGGEST SOME INCREASED FREQUENCY OF SUBAVERAGE INTELLIGENCE QUOTIENTS IN XYY MALES, BUT THE EVIDENCE IS OPEN TO QUESTION. FINALLY, SOCIAL FACTORS STUDIED IN ASSOCIATION WITH AGGRESSION AND THE XYY SYNDROME DO NOT APPEAR TO PLAY A DEFINITE ROLE (THE PRONENESS OF TALLER PERSONS AND PERSONS OF A LOWER ECONOMIC STATUS TO BE INCARCERATED HAS NOT BEEN ESTABLISHED), AND THE INFLUENCE OF FAMILY DYNAMICS AS A FACTOR IN THE DEVELOPMENT OF ANTISOCIAL AND AGGRESSIVE BEHAVIOR IS UNCERTAIN. REFERENCES ARE PROVIDED. (DAG)