OBJECTIVE: This investigation compared the efficacy of the monoamine oxidase inhibitor (MAOI) tranylcypromine with that of the tricyclic imipramine in the treatment of anergic bipolar depressive illness. METHOD: A controlled, double-blind comparison was used to study 56 outpatients who met operationalized criteria for anergic bipolar depression. Patients with bipolar I and II depression were equally distributed between comparison groups. Outcome was measured by the patient-rated Beck Depression Inventory and the clinician-rated Hamilton Rating Scale for Depression, Raskin Mania and Depression Scales, Clinical Global Impression Scale, and the Pittsburgh Reversed Vegetative Symptom Scale. Twenty-eight patients were treated with tranylcypromine and 28 with imipramine. Seventy-three percent of bipolar depressive patients screened for the study met criteria for anergic depression, consistent with previous findings from studies in bipolar illness that stretch back over 100 years. RESULTS: Tranylcypromine produced statistically significant superior outcome in terms of lower attrition, greater symptomatic improvement, and higher global response without increased risk of treatment-emergent hypomania or mania. CONCLUSIONS: The authors propose that the apparently superior efficacy of tranylcypromine in bipolar depression is specifically linked to anergia and reversed neurovegetative symptoms. Bipolar I and bipolar II patients had comparable outcomes, but bipolar I patients had a significantly greater risk of treatment-emergent mood swings. Although the relatively poor showing of imipramine warrants close scrutiny, these findings provide further documentation of the utility of MAOIs in patients presenting with anergia, motor retardation, hyperphagia, and/or hypersomnia.