The XYY constitution has achieved some measure of clinical definition, having been found in association with tallness, asocial personality, dilution of intelligence (Griffiths, 1971) and a tendency to psychopathic disorder (Griffiths, Richards, Zaremba, Abramowicz and Stewart, 1970). In addition, neurological, endocrinological, skeletal and other abnormalities have been reviewed (Griffiths, 1971), and the condition, like mongolism, has come to be regarded as affecting many systems of the body to a greater or lesser extent. Biochemical investigations, hitherto largely concerned with sex hormones, especially testosterone, have in general revealed no significant difference from matched controls. Some authors have, however, reported increased plasma testosterone levels both in 46 XY and 47 XYY patients of maximum security hospitals as compared to patients in other psychiatric hospitals and to the general population (Ismail, Harkness, Kirkham, Loraine, Whatmore and Brittain, 1968; Rudd, Galal and Casey, 1968; Price and Van de Molen, 1970). The pre-eminent desirability of rigorously selected controls has been remarked on in these investigations.