In the present study we tested the hypothesis that individual response to clozapine treatment and/or the occurrence of side-effects may be influenced by genetic variation in the serotonin 5-HT2C receptor. We investigated the frequency of a common Cys23Ser substitution, which is known to alter the pharmacological properties of the protein, in 152 patients treated with clozapine. Presence of the Ser23 variant was previously reported to predict a good response to clozapine. However, our results did not support an association between genetic variation of the 5-HT2C receptor and response to clozapine. This held true whether the patients were subgrouped for sex and length of treatment. Moreover, we found no consistent association with any of the observed side-effects.