To examine the efficacy and tolerability of escitalopram in the prevention of relapse in patients with OCD, 468 patients with OCD were treated with open label escitalopram (10 mg or 20 mg) for 16 weeks, after which the 320 responders (Y-BOCS total score decrease > or =25%) were randomised to placebo or escitalopram (at the assigned dose) for 24 weeks double-blind treatment. The primary analysis (time to relapse) showed a significant advantage for escitalopram (p<0.001, log-rank test). The proportion of patients who relapsed was statistically significantly higher in the placebo group (52%) than in the escitalopram group (23%) (p<0.001, chi(2)-test). The risk of relapse was 2.74 times higher for placebo compared to escitalopram. Escitalopram was well tolerated and improvements in obsessive-compulsive symptoms reported during the open label period were sustained during the double-blind extension of treatment with active drug. These results demonstrate that escitalopram is effective for long-term treatment and relapse prevention in OCD.