BLOOD WAS CULTURED IN THE USUAL WAY FOR 48 TO 72 HOURS, AND A TOTAL OF 10 CELLS WERE ANALYZED FOR THE SAMPLE OF 40 MALES. ALL SUBJECTS WERE GIVEN PSYCHIATRIC AND INTELLIGENCE TESTS AS WELL AS CLINICAL EXAMINATIONS. RESULTS INDICATE THAT 35 OF THE OFFENDERS HAVE A 46, XY KARYOTYPE, 4 HAVE A 47, XYY, AND ONE HAS A 47, XXY. FEW OF THE OFFENDERS HAD ABNORMALLY LOW INTELLIGENCE QUOTIENTS (IQ'S), AND THE XY AND XYY OFFENDERS WERE MOSTLY IN THE 70 TO 85 IQ RANGE. THE XYY OFFENDERS WERE TALL, SHOWED A TENDENCY TOWARD EMOTIONAL LABILITY, WERE IMMATURE WITH OVERDEPENDENCY AND CONTACT NEED, AND HAD DIFFICULTIES WITH THEIR SEX LIVES AND IN MAINTAINING PERSONAL RELATIONSHIPS. OFFENSES COMMITTED BY THE 40 SUBJECTS VARIED AND SHOWED NO CORRELATION WITH CHROMOSOME KARYOTYPES. ALTHOUGH THE XYY OFFENDERS APPEAR TO BE TYPICALLY TALL AND PRONE TO CRIMINAL BEHAVIOR, CONCLUSIONS CANNOT BE MADE UNTIL STUDIES ARE COMPLETED ON CHROMOSOMAL MAKEUP OF THE GENERAL POPULATION. THE SAMPLE IN THIS STUDY AND IN ANOTHER 1968 STUDY CAN BE CONTAMINATED SINCE THE DANISH LEGAL SYSTEM TENDS TO REFER OFFENDERS WITH SUBNORMAL OR BORDERLINE INTELLIGENCE TO CENTERS FOR THE MENTALLY RETARDED. TABULAR DATA AND REFERENCES ARE PROVIDED.