Karotyping of 3011 males at five Wisconsin state correctional institutions revealed that 1% had a chromosome abnormality. The frequency of occurrence of the XYY complement was about 5 times that for newborn males. Approximately the same rate was found among 2556 males in the three penal institutions for adults. The frequency of XYY at the institution for juvenile offenders was about ten times background. The relatively low frequency of XYY (0.38%) found at the mental-penal institution may have been due to previous sampling done there. Of the 16 XYY males discovered, only two were below the 85th percentile for height. A relatively low frequency of XYY was found among black males. Our data contradict the notion that a high rate of XYY among adult males in penal settings may be due to a disproportionately large number of tall men in prisons.