Lithium remains the mainstay of treatment for patients with bipolar affective disorder; however, nearly half of patients with bipolar disorder fail to respond to lithium. Recently, there have been an increasing number of preliminary clinical reports that clozapine, an atypical antipsychotic agent, has potential efficacy in patients with mood disorders. We review the available clinical data supporting the potential use of clozapine in these psychiatric disorders and report our preliminary data from a study that used clozapine in the acute treatment of mania in treatment-refractory patients. Twenty-five patients meeting the DSM-III-R criteria for the manic phase of either bipolar or schizoaffective disorder entered a 13-week open prospective trial of clozapine. These patients either had failed to respond to or had been intolerant to treatment with lithium, an anticonvulsant, and at least two typical neuroleptics. Eighteen of 25 patients demonstrated a greater than 50% decrease in the Young Mania Rating Scale score. These preliminary data as well as the clinical reports reviewed indicate that the efficacy of clozapine in treatment-resistant patients is not limited to patients with schizophrenia.