ALTHOUGH SEVERAL STUDIES OF THE INCIDENCE OF XYY CHROMOSOMAL ABNORMALITIES IN PERSONS CONFINED TO CORRECTIONAL INSTITUTIONS HAVE IDENTIFIED CHARACTERISTICS SAID TO BE COMMON TO INDIVIDUALS WITH KLINEFELTER'S SYNDROME, SUCH AS TALLNESS AND AGGRESSIVENESS, STUDIES INVOLVING INMATES OF MARYLAND'S PATUXENT INSTITUTION HAVE LARGELY FAILED TO SUPPORT THESE OBSERVATIONS. IN SCREENING FOR HEIGHT AND LATER FOR AGGRESSIVE TENDENCY, NO INMATES WERE IDENTIFIED AS XYY'S. ONLY AS A RESULT OF MORE GENERAL SCREENING WERE TWO XYY MALES SUBSEQUENTLY DISCOVERED, AND ALTHOUGH ONE WAS MARGINALLY TALL, NEITHER DISPLAYED OBSERVABLY RETARDED (I.E., LOW INTELLIGENCE QUOTIENT) OR AGGRESSIVE BEHAVIOR. THESE FINDINGS FAIL TO SUPPORT EARLY RESULTS INDICATING AN ASSOCIATION WITH EITHER MENTAL RETARDATION OR AGGRESSIVENESS. THE FREQUENCY OF XYY'S IN THE WHOLE POPULATION ALSO SEEMS SOMEWHAT LESS THAN THAT REPORTED ELSEWHERE, PARTICULARLY SO FOR MENTALLY SUBNORMAL GROUPS. HOWEVER, AN INCREASED LEVEL OF PLASMA TESTOSTERONE WAS NOTED IN ONE OF THE XYY SUBJECTS, ALTHOUGH THERE WERE NO STATISTICALLY SIGNIFICANT DIFFERENCES IN THE MEAN CONCENTRATIONS OF PLASMA TESTOSTERONE IN NORMAL AND XYY INMATES. FOR THE NORMAL SUBJECTS, IT WAS DETERMINED THAT THE TOTAL RIDGE COUNT (TRC) OF WHITE MALE INMATES (EXCLUDING KNOWN XYY'S) WAS SIGNIFICANTLY LOWER THAN THAT FOR THE POPULATION AT LARGE. PERFORMANCE ON VERBAL INTELLIGENCE QUOTIENT TESTS FOR THE PRISON POPULATION WAS FOUND TO BE RELATIVELY HIGH. TABULAR AND GRAPHIC DATA ARE PROVIDED, ALONG WITH REFERENCES. (KBL)