The presence of the 5HT2C receptor allele, Ser23, has recently been reported to predict favorable response to the antipsychotic drug, clozapine. This finding is of interest as Ser23, compared with the more abundant Cys23, alters pharmacological characteristics of the receptor and therefore may provide insights into the mechanism of action of antipsychotic drugs. We determined 5HT2C receptor genotype at the Cys23Ser locus in 66 subjects participating in double-blind studies of clozapine. There was no relationship between Ser23 and clozapine response (p = 0.30, Fisher's exact) nor was there any effect of Ser23 upon absolute levels of psychiatric symptoms after 10 weeks of clozapine treatment (t = -0.57, p = 0.57). These data suggest that this 5HT2C receptor polymorphism is not associated with clozapine response.