Sertraline is a non-tricyclic, potent and selective serotonin reuptake inhibitor (SSRI) which is currently approved for the treatment of depression in several countries, including the UK and the USA. The role of serotonin in the aetiology of obsessive compulsive disorder (OCD) has been established through considerable indirect evidence. The strongest evidence comes from the fact that drugs known to be SSRIs have been found to be useful in the pharmacotherapy of OCD. Two double-blind, placebo-controlled studies, including a total of 412 patients, were undertaken to evaluate the efficacy and safety of sertraline in OCD. The first of these studies of a flexible dosing design showed that sertraline, given for eight weeks in daily dosages of 50-200 mg, was a safe and effective treatment for OCD, and superior to placebo. The second study of a fixed dose design and 12 weeks duration confirmed the efficacy and safety of sertraline in OCD at fixed dosages of 50, 100, or 200 mg/day and demonstrated that further improvement in OCD symptoms can be achieved through continued treatment with sertraline. A comparison between the results of these two studies and similar studies with clomipramine shows that, while both drugs have significant therapeutic efficacy, their side-effect profiles may be markedly distinct.