The serotonergic system has been implicated in both the aetiology and pharmacological treatment of obsessive compulsive disorder. In pharmacological challenge tests, mCPP, a 5-HT agonist, with an affinity for 5HT2C as well as 5HT1A and 5HT1D receptors, was associated with a transient exacerbation of obsessive compulsive symptoms. Chronic administration of mCPP was found to bring about some relief of these symptoms. Sumatriptan is an antimigraine agent, which interacts most potently with 5HT1D receptors. In the cases to be presented, we report the effects of chronic administration of Sumatriptan to three severe, treatment resistant, OCD patients. Following chronic administration of sumatriptan, these patients, who have been resistant to any former pharmacological treatment, responded with an improvement in their depression and a modest reduction in their obsessive compulsive symptoms.