The aim of this prospectively randomized, double-blind, parallel group, multicentre study was to compare the efficacy and tolerability of fluvoxamine and clomipramine in patients suffering from obsessive-compulsive disorder (OCD) (DSM-III-R). Fourteen centres participated in this trial. Sixty-eight patients were randomized to receive fluvoxamine and 65 to receive clomipramine. The duration of the study was 10 weeks. The two treatment groups showed a marked improvement of obsessive-compulsive symptomatology, as determined by the Yale-Brown Obsessive-Compulsive Scale, the National Institute of Mental Health Obsessive-Compulsive Global Scale and Clinical Global Impression. No statistically significant differences were found between fluvoxamine and clomipramine in terms of efficacy during the study. A similar number of patients in each group withdrew from the study prematurely, but there were more dropouts due to adverse events in the clomipramine group. Concerning tolerability, there were significantly more reports of constipation and dry mouth in the clomipramine group. The results show that fluvoxamine and clomipramine have similar efficacy in the treatment of patients suffering from OCD, but fluvoxamine is better tolerated. In view of the superior safety profile of fluvoxamine compared to clomipramine in terms of a risk-benefit assessment, the use of fluvoxamine would appear to be advantageous for this patient population.