Structural brain abnormalities were examined in a sample of 48 patients with bipolar affective disorder who were compared with 54 schizophrenic patients and 47 normal controls. As in our previous work using computed tomographic scanning, lateral ventricular enlargement was due to a diagnostic effect. In this study, the effect was more prominent in the schizophrenic men, while a trend was seen in the bipolar men. Women in both groups did not differ significantly from normal subjects. This finding is possibly consistent with the fact that men have a higher frequency of birth anomalies such as hydrocephalus. Since one cause of such birth anomalies might be periventricular hemorrhage or infarction, we also evaluated all scans for the presence of small focal regions of signal hyperintensity. A significant increase in the number of focal signal hyperintensities was noted in the bipolar patients, in comparison with normal subjects, but not in the schizophrenics. The bipolar patients with focal signal hyperintensities had a trend toward larger ventricular size than those without. The pathophysiological significance of these findings is unclear.