Summary A comparison between 10 mentally deficient individuals with chromatin-positive Klinefelter's syndrome and a control group of mentally defective males showed a much higher incidence of sexually deviant behavior among those with Klinefelter's syndrome. This difference was statistically significant, with a probability of occurrence by chance of less than 0.0025. A sex chromatin survey on 600 institutionalized male sexual psychopaths showed that 6 or 1.0 per cent were chromatin positive. This incidence, while above that of the general population, may have been brought about by certain social factors tending to favor the commitment of men with the abnormalities found in Klinefelter's syndrome. Recent observations in Klinefelter's syndrome indicate that chromatin testing should be done on boys with mental deficiency or with serious personality or behavioral disturbances, especially in those instances when Mongolism or twinning has occurred in the same sibship.