Patients with obsessive-compulsive disorder who met DSM-III criteria and who had been ill for at least one year were studied in a double-blind, randomized, crossover comparison of the tricyclic antidepressant clomipramine hydrochloride and the monoamine oxidase inhibitor clorgyline hydrochloride. No significant improvement was evident after four weeks of treatment with placebo prior to the crossover study. Treatment with clomipramine was associated with significant improvement after both four and six weeks in measures of obsessions, anxiety, and depression. Antiobsessional responses to clomipramine did not depend on presence of depression. Improvement was correlated with plasma concentrations of clomipramine, but not with the plasma concentrations of any of its metabolites. No significant improvement was evident for the entire group with clorgyline treatment, although the conditions of individual patients did respond to the drug.