A number of recent reports have drawn attention to the abnormal EEG findings obtained in subjects with Klinefelter's Syndrome (Dumermuth, 1961; Hambert and S: son Frey, 1964; Hambert, 1964; Nielsen, 1969; Nielsen and Pedersen, 1969; Pasqualini et al., 1957; Prader et al., 1958; Zuppinger et al., 1967). An increased prevalence of epilepsy within this group has also been noted (Hambert, 1964). Much less is known about the electroencephalogram in the XYY syndrome, the available data being limited to single case reports or reports involving only a few subjects (Borgaonkar et al., 1968; Bartlett et al., 1968; Court-Brown et al., 1968; Cowie and Khan, 1968; Forssman, 1967; Forssman and Hambert, 1966; Kessler and Moos, 1970; Leff and Scott, 1968; Mintzer et al., 1968; Nielsen and Pedersen, 1969; Nielsen and Tsuboi, 1969; Nielsen et al., 1966; Pergament et al., 1968; Persson, 1967; Weiner et al., 1968; Welch et al., 1967). This paper describes the initial findings of a comparative study of Special Hospital patients with sex chromosome abnormalities and a group of matched controls of normal chromosome complement from the same institution.