IT is well known that 1 per cent of males in institutions for the mentally sub-normal are chromatin-positive and that the majority of these have an XXY sex chromosome constitution1. In 1963 Forssman and Hambert2 reported on a survey of the nuclear sex of 760 male patients in three Swedish institutions for criminal and `hard-to-manage' males of sub-normal intelligence. They found 2 per cent to be chromatin positive, but the chromosome constitution of these patients was not reported. Casey (personal communication) surveyed 942 males in two comparable institutions in England, and found 2.2 per cent of them to be chromatin-positive. He also studied the chromosomes of these 21 chromatin-positive males and found 7 to have either an XXYY sex chromosome constitution or to be mosaics in which the XXYY cells formed the major cell line. This finding of a third of the chromatin-positive males to have an XXYY cell line contrasted sharply with that of Maclean and his colleagues1, who studied 2,607 ordinary mentally sub-normal males, and found only 2 of the 28 chromatin-positive patients to have an XXYY constitution. This contrast led us to wonder whether an extra Y chromosome predisposes its carriers to unusually aggressive behaviour. We decided that if this were the case, then we might expect an increased frequency of XYY males among those of a violent nature. The purpose of this communication is to report our findings in a survey of mentally sub-normal male patients with dangerous, violent or criminal propensities in an institution where they are treated under conditions of special security.