A high incidence of men with the 47 XYY karyotype has been repeatedly demonstrated amongst inmates of prisons (Bartlett et al., 1968; Griffiths et al., 1967) and institutions of psychologically abnormal offenders (Jacobs et al., 1968; Casey et al., 1968). Although genetic surveys of the newborn suggest that there must be many more males with this karyotype who remain undetected in the general population, the high incidence of this karyotype amongst these highly selected groups remains unexplained. The initial hypothesis that the extra Y chromosome was associated with aggressiveness no longer appears tenable, at least in its extreme form, as studies indicate that men with this karyotype are less aggressive than other men in the same institution (Price and Whatmore, 1967). It has also been suggested (Hunter, 1966) that because of their increased stature they are more likely to be seen as aggressive and dangerous, and that emotional disturbances arise from problems consequent to being tall. Forssman and Hambert (1969) have suggested that some cerebral abnormality might account for the behaviour of these individuals, and point to reports of EEG abnormalities as being in keeping with this. In support of this idea, Daly (1969) has reported finding neurological abnormalities in 10 out of 12 XYY males that he examined. He further suggested that neurological deficits might prove valuable in identifying individuals of this karyotype. Criticism has been made of such conclusions (Kessler and Moos, 1970) on the grounds that all such studies have been carried out on patients in special institutions, and that the prevalence of similar abnormalities in the rest of these populations is not known. The following report describes neurological findings in 23 XYY men and in a group of patients with a 46 XY karyotype from the same institution, matched in respect of age (6 months), intelligence (15 points on WAIS), and length of stay (within two years). All patients were from Rampton and Moss Side Hospitals (two of the English Special Hospitals) and had been karyotyped in a previous survey (see Casey, 1971). Apart from those on anticonvulsant medication, all patients had been off drugs for at least the preceding four weeks. Controls and index cases were seen in a randomized order by one of us (M.J.G.H.) who did not know at the time of examination to which group the patient belonged. The neurological anomalies detected in the two groups are shown in the table below.