Recent epidemiological data have shown that obsessive compulsive disorder (OCD) has a lifetime prevalence of 2.5–3.0%. Compounds having an effect on serotonergic neurotransmission have demonstrated a reproducible therapeutic effect in OCD. Most recent attention has been focused on the specific serotonin re-uptake inhibitor (SSRI) class of compounds. The pooled data from two recently completed placebo-controlled studies of the SSRI, fluoxetine, have demonstrated clear evidence of a clinically valuable therapeutic effect in patients with OCD. Combined efficacy and safety data suggest that fluoxetine, 20 mg/day, will be effective in many patients. The therapeutic response emerges slowly and increases gradually. Data from extension studies suggest that, in some patients, an optimal response may only be achieved with higher daily doses of fluoxetine. Clinical response has been shown to be sustained over periods of 9 months of treatment and following withdrawal in an open-label trial. Fluoxetine is safe and well tolerated in this group of patients.