A great deal of evidence suggests that a genetic component underlies obsessive-compulsive disorder (OCD). The response to serotonergic medications and the worsening of obsessive symptoms after administration of serotonergic agonists indicate that serotonergic mechanisms are involved in OCD. We investigated the role of the Cys23Ser mutation of the 5HT2C receptor gene in the etiology of this disorder by performing an association study comparing a sample of 109 OCD patients with a sample of 107 healthy control subjects. No allelic or genotypic association of OCD with the 5HT2C receptor gene mutation was revealed in our data. We also extended the association analysis to a subsample of 39 OCD patients that had previously been submitted to a challenge test with clomipramine. In the subsample of OCD patients that received the challenge with clomipramine, no association between the 5HT2C receptor gene mutation and response to the challenge test was found. Our results exclude any specific role of the Cys23Ser mutation of 5HT2C receptor gene in the etiology of OCD: it seems probable that more complex genetic models are needed to explain the involvement of serotonergic elements in the etiology of this disorder.