Approximately one half of the electrocardiograms of 106 hospitalized psychiatric patients showed the characteristic T-wave changes ascribed to thioridazine hydrochloride. These were more frequent in the female patients and were not related to aging or to the psychiatric diagnosis. Repeat ECGs three months later in 68 of these patients excluded a time-dosage effect and demonstrated some fluctuation of the appearance of the T wave during constant dosage. The U wave does not account for the notched T wave. The survey suggested that the average duration of repolarization varied directly with the degree of morphologic change but further study of 18 patients disclosed that the effect of thioridazine or chlorpromazine on the T wave is distinct from their effect on the duration of repolarization. The study shows that the electrocardiographic changes are benign.