Electroencephalograms of 232 patients in mental hospitals, including 180 unselected cases at McLean Hospital and 52 chronic schizophrenics at Metropolitan State Hospital, have been studied since October, 1936. The fundamental patterns of these patients cannot be distinguished from those of our control series of 500 " normals." There is, however, a large proportion of patients whose records show variations outside of the normal range of variability (see Figs. 3 and 10). The abnormalities which break into the pattern, rather than the type of pattern itself, constitute the significant difference between the electroencephalograms of these patients and of normal individuals.The quality of the atypical variations resembles that of the changes which occur in the electroencephalograms of persons known to have brain lesions, or who suffer from the various forms of epilepsy (Fig. 9), or of normal persons during sleep (Fig. 5), or when breathing an inadequate supply of oxygen (Fig. 4).All the patients clinically diagnosed as having epilepsy or " psychosis with associated cerebral arteriosclerosis" showed abnormal variations, but it is significant that equally abnormal variations are distributed throughout all the other diagnostic groups.Abstract Teaser