Rationale: Conventional as well as newer antipsychotics cause weight gain, and, in the regulation of body weight, both insulin and leptin are hormones involved. Objective: The aim of the present study was to compare these hormonal levels in patients on treatment with different antipsychotics. Methods: Nineteen patients receiving conventional antipsychotics, 14 patients receiving clozapine and 14 patients receiving olanzapine, were studied. Fasting blood samples for insulin, leptin, glucose, and drug serum concentrations were analyzed. In addition, body mass index (BMI) was calculated. Results: The median insulin level was significantly higher in the patients receiving olanzapine than in those receiving conventional agents, whereas there was no significant difference in insulin between the clozapine and the other two groups. However, in the clozapine group, insulin levels were positively correlated to the drug serum concentration. BMI was elevated in about half of the patients, with no difference being found between the groups. The leptin level was significantly higher in the women than in the men in the conventional agent group, but not in the olanzapine or clozapine groups. Conclusions: The higher insulin level in the patients receiving olanzapine than in those receiving conventional antipsychotics, despite similar BMI, points to a probable influence of olanzapine on insulin secretion. The correlation between the insulin levels and the clozapine concentration indicates, in addition, an influence of clozapine on insulin secretion. The gender difference in leptin, i.e. females normally having higher leptin levels than males, was found in the conventional agent group, but not in the olanzapine or clozapine groups, suggesting that also leptin regulation is altered during olanzapine or clozapine treatments. Moreover, it was mainly due to an increase of leptin in the males that leptin levels were equalized between sexes in the olanzapine group. We conclude that the influence of olanzapine and clozapine on both insulin and leptin levels might be associated with their weight-gain-inducing ability, while other mechanisms may be involved in the weight gain caused by conventional antipsychotics.