• A six- to eight-week double-blind placebo-controlled trial of the potent and selective serotonin reuptake inhibitor fluvoxamine was conducted in 42 patients with primary obsessive-compulsive disorder (OCD). Approximately one half of the patients also had symptoms of major depression. Fluvoxamine was significantly better than placebo on all measures of obsessive-compulsive symptoms. Nine of 21 patients were responders ("much improved") with fluvoxamine compared with no responders with placebo, and fluvoxamine was effective in patients with OCD both with and without secondary depression. Response of OCD was not correlated with severity of baseline depression. These data lend partial support to the serotonin hypothesis of OCD. However, since a number of patients failed to respond to fluvoxamine, the role of other neurochemical systems in this disorder needs to be explored.