Objective:The purpose of this study was to know about the mechanism of pathogenesis of type 2 diabetes mellitus by using of blood glucose, glucoregulatory factor, insulin resistance in schizophrenic patients receiving antipsychotics. Method:Modified oral glucose tolerance tests were performed in 20 schizophrenic patients receiving haloperidol, risperidone and olanzapine. Insulin, glucagon, C-peptide and cortisol were measured in 0, 15, 45, 75 minutes after glucose loading, and insulin resistance was calculated by HOMA(homeostasis model assessment) method. Result:Olanzapine-treated patients had significant glucose elevation 45 minutes after glucose challenge. Also modest increases in HOMA IR values were detected in patients treated with olanzapine. Conclusion:Olanzapine treatment of non-diabetic patients with schizophrenia can be associated with type 2 diabetes mellitus through the elevation of glucose and insulin resistance. Elevated insulin resistance may be a causative mechanism of type 2 diabetes mellitus in patients receiving olanzapine.