OF 2,200 INMATES IN A FEDERAL PRISON, 506 OVER 6 FEET IN HEIGHT WERE SELECTED FOR EVALUATION. (TALLNESS IS ASSOCIATED WITH THE XYY KARYOTYPE). A PSYCHIATRIST REVIEWED THE RECORDS OF THE 506 INMATES AND SELECTED FOR FURTHER STUDY 245 WHOSE HISTORIES INCLUDED RECURRENT CRIMINALITY BUT NO EVIDENCE OF PSYCHOSIS OR ORGANIC BRAIN DAMAGE. OF THESE, 22 WITH INTELLIGENCE SCORES BETWEEN 70 AND 80 RECEIVED CHROMOSOME CHECKS. A PSYCHOLOGIST ADMINISTERED THE GROUP FORM OF THE MINNESOTA MULTIPHASIC PERSONALITY INVENTORY (MMPI) TO 85 SUBJECTS WITH INTELLIGENCE SCORES OF 80 OR ABOVE. THE MMPI SCORES OF 83 WERE EVALUATED, AND 3 SUBJECTS WHOSE SCORES INDICATED THEY WERE PSYCHOPATHIC RECEIVED CHROMOSOME TESTS. NO XYY CHROMOSOME PATTERN WAS FOUND AMONG THE 25 SUBJECTS WHOSE HEIGHT AND PSYCHIATRIC AND OR PSYCHOLOGICAL TRAITS THOUGHT TO BE CHARACTERISTIC OF THE XYY ANOMALY. A COMPLETE CASE STUDY OF THIS SUBJECT IS REPORTED. THE FINDINGS INDICATE THAT, PRIOR TO ANY FURTHER SPECULATION ON THE RELATIONSHIP BETWEEN THE XYY PATTERN AND PSYCHOPATHIC CRIMINAL BEHAVIOR, A CHROMOSOME SURVEY OF ALL INMATES ADMITTED TO A PENAL INSTITUTION OVER A 1-YEAR PERIOD SHOULD BE CONDUCTED. DIAGNOSTIC TOOLS COULD BE DEVELOPED ON THE BASIS OF MEDICAL, SOCIAL, PSYCHIATRIC, AND PSYCHOLOGICAL EVALUATIONS OF THOSE INMATES FOUND TO HAVE THE XYY CHROMOSOME COMPLEMENT. SUPPORTING DATA AND A LIST OF REFERENCES ARE INCLUDED. (LKM)