OBJECTIVE: Weight gain, leading to further morbidity and poor treatment adherence, is a common consequence of treatment with antipsychotic drugs. A recent study showed that a polymorphism of the promoter region of the serotonin 5-HT2C receptor gene is associated with antipsychotic-induced weight gain. The authors determined whether this association held true for weight gain after clozapine treatment. METHOD: Thirty-two Chinese Han patients with first-episode schizophrenia were genotyped for the –759C/T polymorphism and had weight changes monitored after 6 weeks of clozapine treatment. RESULTS: The authors found that the 10 patients with the –759T variant allele showed significantly less weight gain than those without this allele. The effect was strongest in the male patients and not apparent in the female patients. CONCLUSIONS: These findings identify an important genetic factor associated with clozapine-induced weight increases in schizophrenia.