The long-term treatment outcome of 248 bipolar patients in an out-patient lithium programme was assessed. Over half of the patients (138 or 56%) had no affective episodes in the year observed. Patients were divided into outcome groups according to GAS scores: the outcome for 40% of patients was good, for 41% fair, and for 19% poor. More frequent psychiatric admissions before starting lithium treatment was the best predictor of poor outcome, followed by a negative affective style in the family and lower social class. Current alcohol and drug abuse was associated with poor outcome. Although familial and psychosocial factors were significantly associated with outcome, the findings suggest there may be inherent differences in the pathophysiology of bipolar disorder reflected in an increased frequency of episodes which account for a large variance in lithium treatment outcome.