Of the 241 lithium clinic patients at the New York State Psychiatric Institute with bipolar I affective disorder, 38 (15.7%) had never been hospitalized or somatically treated for depression. These "unipolar manic" patients had a significantly lower incidence of rapid cycling and suicide attempts than other bipolar I patients. No differences were found, however, in risk of illness in first-degree relatives. Lithium was an effective prophylactic agent in these patients. Some patients originally classified as "unipolar manic" were found to have depressive episodes with additional information and clinical observation. "Unipolar mania" appears to be a subgroup of bipolar I illness, but there are no data to support the hypothesis that it is a separate entity.