Several studies have suggested that risperidone is superior to olanzapine in glucose tolerance; however, there is little information available about the risk of impaired glucose metabolism induced by atypical antipsychotics in the same patients. A 75-g oral glucose tolerance test (OGTT) was performed in 22 mildly obese, diabetes-free, Japanese patients with schizophrenia who received risperidone or olanzapine for at least 2 months. After the OGTT, the medication was switched to another by decreasing the previous dosage gradually over 2 to 8 months after the initiation of the second medicine. After at least 8 weeks of complete switching, the same OGTT procedure was conducted. Fasting insulin concentration and the homeostasis model assessment of insulin resistance (HOMA-IR) during olanzapine treatment were significantly higher compared with risperidone treatment. The area under the concentration-time curves of serum insulin concentrations from 0 to 120 min was different in patients receiving risperidone compared with patients receiving olanzapine; however, there were no differences in the insulinogenic index between the two groups. The present study suggests that olanzapine might impair glucose tolerance to some extent because of an increase in insulin resistance compared with risperidone.