A double-blind trial of phenelzine versus placebo in psychiatric outpatients with long-standing phobic anxiety is described. Forty patients with agoraphobia or social phobias entered the trial and 32 of these completed treatment with either phenelzine or placebo (with appropriate dietary restrictions) in flexible dosage for two months. Fourteen pairs of patients were prospectively matched, and analysis of the score differences between these pairs showed a significant improvement for patients on phenelzine compared with placebo on overall assessment (P<0·01) and secondary phobias (P<0·05). These differences were not shown at assessment after one month's treatment, but became marked after two months. The improvement on phenelzine, which was maintained at follow-up, was not related to initial depressive symptoms in the patients. It is suggested that phenelzine is an effective agent in the treatment of phobic anxiety but that treatment needs to be continued for at least two months before full benefit is achieved.