IN 1965, Jacobs et al. published their preliminary findings of a chromosome survey conducted at a maximum security hospital, The State Hospital, Lanarkshire, Scotland1. The most remarkable finding in the completed survey was the discovery among 315 men of nine patients with an XYY sex chromosome constitution. Their behaviour, together with their pattern of crime, has now been closely studied. The full clinical details of this investigation will be published elsewhere by us, and this communication directs attention to the ways in which the XYY males differ from males with an XY sex chromosome complement at the same hospital.