For 51 patients with rapid cycling affective disorder, clinical and family history data indicated that the illness was phenotypically and genetically related to more typical forms of affective disorder, was characterized by a bipolar course (100%), and was more common in women (92%). Manic-depressive cycles were separate from menstrual cycles. At the time of onset of rapid cycling, 73% of the patients were taking antidepressant drugs; the continuation of rapid cycling was associated with antidepressant drug therapy in 51% of the patients. Although most patients had been referred to a research ward because they were considered to be refractory to treatment, 37% attained essentially complete remissions, usually during treatment with lithium and/or monoamine oxidase inhibitors.