Obsessive-compulsive disorder (OCD) is emerging very clearly as a serotonin specific illness. The evidence for this comes from a variety of clinical sources. Firstly the efficacy of 5-HT uptake inhibitors, especially clomipramine, is consistent and strong. Secondly clomipramine has not merely been found to be effective against placebo in 9 placebo controlled studies, it is also found to be more effective in some studies than reference tricyclic antidepressants and monoamine oxidase inhibitors. In other studies tricyclic antidepressants do not appear to be better than placebo. The lack of efficacy of general antidepressants, neuroleptics and benzodiazepines supports the specificity of the serotonin effect. Thirdly the evidence of the efficacy of clomipramine in OCD without concomitant depression reported by Montgomery 1980 and supported by other studies suggests that 5-HT uptake inhibitors have a specifically anti-obsessional effect. The lack of response of depressive symptoms in OCD to other antidepressants suggests that these depressive symptoms are integral to OCD and will only respond when the OCD is treated. Many of the studies found efficacy for 5-HT uptake inhibitors compared with placebo despite both groups being treated with concomitant behaviour therapy. This argues either for behaviour therapy being relatively ineffective or for there being a synergistic effect with the 5-HT uptake inhibitor which is more likely. Results from studies with selective serotonergic probes with a worsening of OCD symptoms in response to the 5-HT1A agonist, m-chlorophenyl piperazine, add support to the serotonergic hypothesis of OCD but further investigation is needed.