The efficacy and tolerability of fluvoxamine (100-300 mg/day) and clomipramine (100-250 mg/day) were compared in a randomized, double-blind, parallel-group study of 79 patients with obsessive-compulsive disorder (OCD) without coexisting major depression. After a 2-week placebo lead-in period, patients were randomized to fluvoxamine (37 patients) or clomipramine (42 patients) for 10 weeks. Efficacy was evaluated with the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), the National Institute of Mental Health Obsessive-Compulsive scale, and Patient and Clinical Global Improvement scales. Hamilton Rating Scale for Depression scores and somatic symptoms were also assessed. Seventy-eight percent of fluvoxamine patients and 64% of clomipramine patients completed the study. At the end of treatment, 56% of fluvoxamine patients were classified as responders (greater or equal to 25% decrease in Y-BOCS score), compared with 54% of clomipramine patients. Both groups showed steady improvement throughout the study; no statistically significant differences were observed between the groups for any efficacy variable at any time. A similar percentage of patients in both groups withdrew because of adverse events. No serious adverse events related to drug occurred with either drug. Insomnia, nervousness, and dyspepsia were more statistically frequent with fluvoxamine; dry mouth and postural hypotension were more frequent with clomipramine. In this study, fluvoxamine and clomipramine were equally effective in reducing OCD symptoms over a 10-week treatment period but displayed different side effect profiles. (J Clin Psychopharmacol 1996;16:121-129).