Convincing evidence from placebo-referenced randomized controlled trials supports efficacy for clomipramine and selective serotonin reuptake inhibitors for acute treatment of obsessive–compulsive disorder. It remains less conclusively understood whether these agents maintain efficacy over the longer term. This paper systematically reviews long-term medication studies in obsessive–compulsive disorder. Studies of clomipramine, fluoxetine and sertraline investigated ‘responders’ from acute treatment trials and extended treatment up to 12 months versus placebo. Responses to medication were sustained. A 24-week placebo-controlled trial of escitalopram (10 mg or 20 mg/day) and paroxetine (40 mg/day) demonstrated ongoing efficacy for all three treatments. Studies that randomized treated cases to placebo demonstrated reemergence of symptoms in the placebo-treated cohort. Six relapse prevention trials were found by systematic search. Some, but not all, revealed significant advantages for remaining on medication. Paroxetine (20–60 mg/day) and escitalopram (10 or 20 mg/day) were each found to outperform placebo in preventing relapse during 24 weeks of double-blind, randomized follow-up. Meta-analysis, using Review Manager software (4.2.8), detected overall superiority of selective serotonin reuptake inhibitors to placebo in preventing relapse among adult treatment-responders. Worsening by five Yale–Brown Obsessive Compulsive Scale points emerged from the review as a suggested threshold for relapse. Viewed collectively, these results suggest that selective serotonin reuptake inhibitors are effective long-term treatments and relapse prevention represents the treatment target for obsessive–compulsive disorder.