THE 48,XXYY KARYOTYPE, THE KLINEFELTER SYNDROME WITH AN ADDITIONAL Y CHROMOSOME, IS SUFFICIENTLY UNCOMMON TO WARRANT DOCUMENTATION. THE MALE CARRYING THE 48,XXYY KARYOTYPE WAS REFERRED TO THE HOSPITAL BY HIS PARENTS, WHO COMPLAINED OF HIS AGGRESSIVE AND ARGUMENTATIVE BEHAVIOR. HE HAD SUFFERED BRAIN DAMAGE AT THE AGE OF THREE AND DID SO POORLY IN SCHOOL THAT EVENTUALLY HE HAD TO BE ENROLLED IN SPECIAL SCHOOLS. HE HAD SHOWN VIOLENCE TOWARD SCHOOL FRIENDS AND FAMILY SERVANTS. THE MAN IS TALL, THIN, WITH A NORMALLY SHAPED BODY, EXCEPT FOR A SLIGHT ROUNDING OF THE HIPS. AN ELECTROENCEPHALOGRAM STUDY DISCLOSED OCCASIONAL EPISODIC 0-WAVES IN BOTH TEMPORAL AREAS. HIS TOTAL INTELLIGENCE QUOTIENT, ASSESSED ON THE WECHSLER ADULT SCALE, WAS 52, BUT THE LOW SCORE WAS PARTLY DUE TO THE UNCOOPERATIVENESS OF THE SUBJECT. THE SUBJECT APPEARED TO POSSESS SOME, BUT NOT ALL, CHARACTERISTICS ASSOCIATED WITH KLINEFELTER'S SYNDROME, INCLUDING TALLNESS, TESTICULAR ATROPHY, DECREASED UPPER TO LOWER BODY RATIO, AND MENTAL SUBNORMALITY. REFERENCES AND PHOTOGRAPHS OF THE SUBJECT AND THE KARYOTYPE ARE PROVIDED. (DAG)