A total of 876 males in prisons and facilities for the mentally ill and mentally retarded underwent cytogenetic screening. Of the 23 individuals identified with sex chromosome errors, seven with 47,XYY chromosome pattern and eight with Klinefelter's syndrome were compared. Both groups displayed tall stature and elongated lower segments; the XYY males averaged 3 inches taller than the Klinefelter males. In contrast to 47,XYY males, Klinefelter males uniformly displayed atrophic testes and buccal smears positive for sex chromatin. Facial acne, ulcerous varicosities of the legs, and neurological disorders were inconsistent features of both groups. The Klinefelter males of this series displayed a mean intelligence quotient of 80 as opposed to 84 for the 47,XYY males. A wide spectrum of criminal offenses and psychopathology, including psychosis, was manifested in both groups.