In a survey of male patients admitted to the four maximum security hospitals in the United Kingdom during 1972 and 1973, 26 out of 611 (4.26%) karyotyped were found to have chromosome abnormalities. Of these, 13 (2.13%) had a 47,XYY abnormality; 5 (0.8%) were chromatin positive but two of these had cell lines that included an extra Y chromosome, one being a 48XXYY male and the other a mosaic 48,XXYY/XYY. Eight patients with autosomal abnormalities included two with Down's syndrome and a 47,XY,+21 karyotype, two with small supernumerary chromosomes, 47,XY+mar, and two with inherited balanced translocation. As in the prevalence studies of patients in these high secruity hospitals, the significant finding was the very high frequency of males with extra Y chromosomes. In this survey, it was most marked in the younger patients and particularly in those who were aged 20 years or less (approximately 6%). As a group the XYY males were therefore significantly younger than all other male admissions. As in previous surveys thay were also significantly taller and 60% were 185 cm or more in height. There were no other distinguishing physical characteristics associated with the 47,XYY karytype. In all but two patients the behaviour disturbances were attributable to psychopathic disorders and mental subnormality was diagnosed no more frequently than in the total population studied (approximately 50%).