THE FIRST EVIDENCE ASSOCIATING XYY CHROMOSOME MALES WITH ANTISOCIAL BEHAVIOR WAS GATHERED IN STUDIES IN THE 1960'S. RESEARCHERS LATER MADE INTENSIVE EFFORTS TO LOCATE XYY MEN IN APPARENTLY 'HIGH RISK' SUBPOPULATIONS IN PENAL INSTITUTIONS AND MENTAL HOSPITALS, AND THEY ARE INCREASINGLY CONDUCTING STUDIES TO ASSESS AND COMPARE A VARIETY OF PHENOTYPIC CHARACTERISTICS (PHYSICAL, NEUROLOGIC, ENDOCRINOLOGIC, AND BEHAVIORAL) USING CAREFULLY MATCHED CONTROLS FROM SIMILAR SUBPOPULATIONS AND APPLYING NECESSARY DOUBLE-BLIND PROCEDURES. A MAJOR PROBLEM HAS BEEN POPULATION SAMPLING ON A SCALE LARGE ENOUGH TO VALIDATE FINDINGS. A REVIEW OF XYY STUDIES TO 1974 SUGGESTS THAT THE EXTRA Y CHROMOSOME IS SIMILAR IN STRUCTURE AND FUNCTION TO THE FIRST Y; THE XYY CONDITION IS PROBABLY PRESENT IN 1 IN 1,500 TO 1 IN 3,000 MALES; XYY MALES ARE SOMEWHAT TALLER THAN AVERAGE; AND XYY'S ARE MORE PREVALENT AMONG GROUPS INSTITUTIONALIZED FOR AGGRESSIVE OR ANTISOCIAL BEHAVIOR THAN AMONG THE GENERAL POPULATION. INCONCLUSIVE EVIDENCE HAS RESULTED IN ADDITIONAL STUDIES ON SKELETAL, DERMATOLOGIC, ENDOCRINOLOGICAL, NEUROLOGICAL, BEHAVIORAL, PSYCHOLOGICAL, AND DEMOGRAPHIC CHARACTERISTICS OF MALES WITH XYY KARYOTYPES. DESPITE THE UNCERTAINTY OF A VALID ASSOCIATION BETWEEN THE KARYOTYPE AND PROBLEM BEHAVIOR, SIX CRIMINAL TRIALS HAVE OCCURRED IN WHICH THE DEFENDANT HAS PUT FORWARD THE XYY CHROMOSOMAL CONDITION AS A BASIS FOR AN INSANITY PLEA. IMPLICATIONS ARE DISCUSSED FOR SPECIAL HANDLING, PREVENTIVE DETENTION, CRIMINAL SENTENCING, PAROLE DECISION, GENETIC COUNSELING, CONFIDENTIALITY, AND INFORMED CONSENT REGARDING XYY'S. REFERENCES AND TABULAR DATA ARE PROVIDED. (DAG)