Based on clinical similarities with schizophrenics and previous computed tomography (CT) studies that found distinct structural abnormalities in the brains of bipolar patients, we evaluated 26 DSM-III bipolar patients and 22 controls by CT, using quantitative measures of ventricular and sulcal size and of cerebral parenchymal density. Third ventricle size was increased, as was periventricular and cortical density. Comparison is made with results found in other psychotic conditions and the possible etiopathological significance discussed.