The authors describe the use of monoamine oxidase inhibitors (MAOIs) in eight patients with severe obsessive-compulsive disorder. In four cases, the MAOIs produced rapid and sustained remission of symptoms; no response was seen in four other patients. All patients who responded to MAOIs, but none of the nonresponders, had phobic anxiety and/or panic attacks. Thus, a trial of MAOIs is indicated in obsessive-compulsive disorder, especially when phobic anxiety or panic attacks form part of the clinical picture.