BACKGROUND To examine the efficacy of fluvoxamine and clomipramine in obsessive compulsive disorder and to compare their tolerabilities. METHODS In this multicenter, randomized, double-blind trial, fluvoxamine (100-250 mg/day) was compared with clomipramine (100-250 mg/day) for 10 weeks in the treatment of 66 psychiatric outpatients, aged 18 to 65 years, with a diagnosis of obsessive compulsive disorder. The main efficacy variable was the Yale-Brown Obsessive Compulsive Scale; secondary variables were the National Institute of Mental Health Global Obsessive Compulsive Scale and the Clinical Global Impressions-Improvement scale. RESULTS Seventeen patients withdrew prematurely, 6 in the fluvoxamine group and 11 in the clomipramine group. In the intent-to-treat population (34 fluvoxamine patients and 30 clomipramine patients), there were no significant differences with respect to the mean reduction in total Yale-Brown Obsessive Compulsive Scale score (last observation carried forward) at any time-point; a mean reduction of 8.6 (33%) was seen in the fluvoxamine group and 7.8 (31%) in the clomipramine group. Similar results were obtained in virtually all secondary variables. The only exception was the obsession-free interval for the Yale-Brown Obsessive Compulsive Scale, which was significantly longer in the fluvoxamine group, especially in a population of patients with disease of > 12 months' duration (F = 5.298, df = 1, p = .026). Adverse events were mostly tolerable; 9 patients (5 receiving fluvoxamine, 4 receiving clomipramine) withdrew due to adverse events related to treatment. CONCLUSIONS Fluvoxamine and clomipramine were equally effective in the treatment of obsessive compulsive disorder. Both agents were well tolerated; fluvoxamine produced fewer anticholinergic side effects and caused less sexual dysfunction than clomipramine, but more reports of headache and insomnia.