Four new examples of the XYY karyotype were ascertained in a survey of selected male populations. Among men 183 cm (6 ft) or greater in height, two out of 86 tested in a state prison, one out of 76 tested in a mental hospital, and none in 30 normal men were found to be XYY. One 11-year-old boy was also found among a group of 57 male juvenile offenders not selected for height, referred from a family court clinic. None of the XYY subjects showed significant physical abnormalities other than increased height. All came from broken or unsatisfactory homes, perhaps an important factor in leading to their sociopathic behavior.