THE INVESTIGATION INVOLVED OBTAINING BLOOD SAMPLES OF 135 OF THE TALLEST AND MOST IMMATURE OF 217 INMATES OF HORSENS INSTITUTIONS FOR CRIMINAL PSYCHOPATHS. AN INCIDENCE STUDY OF PRISONERS ADMITTED TO THE INSTITUTION FROM OCTOBER 1967 TO OCTOBER 1968 WAS ALSO MADE. CHROMOSOME EXAMINATIONS WERE MADE ON 97 OF THESE MEN. PREVALENCE AND INCIDENCE FIGURES WERE CALCULATED ON THE TOTAL NUMBER OF PATIENTS (325), NOT ONLY ON THOSE EXAMINED. SEX CHROMATIN ANALYSIS WAS MADE ON ALL PATIENTS WITH KARYOTYPE 47, XXY, USING THE FEULGEN STAINING METHOD. THE RESULTS OF THE CHROMOSOME ANALYSIS IN THE 10 PATIENTS WITH CHROMOSOMAL ABERRATIONS SHOW THAT 5 PATIENTS HAVE KLINEFELTER'S SYNDROME 47, XXY AND SEX CHROMATIN-POSITIVE CELLS, AND 3 HAVE THE 47, XYY AND SEX CHROMATIN-NEGATIVE CELLS. THE FREQUENCY OF KLINEFELTER'S SYNDROME WAS 1.5 PERCENT AND THE FREQUENCY OF THE XYY SYNDROME WAS 0.9 PERCENT FOR ALL PATIENTS. ALL FREQUENCIES WERE SIGNIFICANTLY HIGHER THAN EXPECTED IN THE GENERAL MALE POPULATION. CHROMOSOME ABNORMALITIES WHICH HAD NOT BEEN PREVIOUSLY DESCRIBED WERE APPARENT IN TWO PATIENTS. THE AGE OF FIRST CRIMINALITY, THE TYPE OF CRIMINALITY, CRIMINALITY AMONG RELATIVES, ENVIRONMENT, INTELLIGENCE, AND HISTORY OF MENTAL ILLNESS IS DESCRIBED FOR 10 PATIENTS WITH CHROMOSOME ABNORMALITIES. NONE OF THE PATIENTS WERE MENTALLY RETARDED. CRIMINALITY AGAINST PROPERTY IS THE MOST FREQUENT OFFENSE COMMITTED BY KLINEFELTER AND XYY MALES, ALTHOUGH A COMPARATIVELY HIGHER FREQUENCY OF SEXUAL CRIMINALITY WAS FOUND AMONG KLINEFELTER AND XYY MALES THAN THE REST OF THE SAMPLE. VIOLENCE AND ARSON OFFENSES WERE MORE FREQUENT FOR XYY PATIENTS THAN FOR KLINEFELTER PATIENTS. PATIENTS WITH KLINEFELTER'S SYNDROME ARE MORE IMMATURE, SENSITIVE, AND PASSIVE, AND XYY'S ARE MORE AGGRESSIVE, HAVE A MORE PRONOUNCED SEXUAL LIBIDO, AND HAVE DIFFICULTY IN INTERACTING WITH OTHER PERSONS. REFERENCES AND STUDY DATA ARE PRESENTED. (DAG)