OBJECTIVE: Leptin is produced by fat cells and is presumed to signal the size of the adipose tissue to the brain. The authors investigated whether antipsychotic drugs that often induce weight gain affect circulating levels of leptin. METHOD: Weight, body mass index, and leptin plasma level were measured weekly over 4 weeks in psychiatric inpatients who received clozapine (N=11), olanzapine (N=8), haloperidol (N=13), or no psychopharmacological treatment (N=12). RESULTS: In patients receiving clozapine or olanzapine, significant increases in weight, body mass index, and leptin level were found, whereas these measures remained stable in patients who received haloperidol or no pharmacological treatment. CONCLUSIONS: Weight gain induced by clozapine or olanzapine appears to be associated with an increase in leptin level that cannot be attributed to dietary changes upon hospitalization.