Studies that compare the treatment response of patients diagnosed as primary affective disorder or schizoaffective disorder are reviewed. Although relatively few controlled or uncontrolled studies of the chemotherapy of schizoaffective disorders have been conducted, available evidence suggests that: (1) lithium carbonate is effective in the initial treatment of both schizoaffective mania and mania; (2) antidepressants alone, neuroleptics alone, or their combination can be effective in the initial treatment of both schizoaffective depression and primary depression; and (3) prophylactic administration of lithium carbonate may reduce the frequency and duration of relapse in both schizoaffective manic and schizoaffective depressed patients. Thus, treatment studies indicate that the schizoaffective disorders are very similar to the primary affective disorders with regard to response to pharmacologic treatment. Evidence from this laboratory that schizoaffective manic patients respond more slowly than manic patients to lithium or neuroleptic treatment is presented.