Magnetic resonance imaging was utilized to determine the nature and rate of subcortical abnormalities in bipolar affective disorder. Nine of 19 bipolar patients and no controls demonstrated subcortical signal hyperintensities on blind evaluation of the images. There was no apparent change in the appearance of the hyperintensities in 7 of 7 subjects with abnormal magnetic resonance images who underwent repeated imaging at 1 year. Bipolar patients with abnormalities had a history of more hospitalizations and appeared more impaired on tests of fluency and recall when compared with bipolar patients without abnormalities or with controls. The possible etiology and significance of signal hyperintensities in bipolar affective disorder is discussed.