Recently, increasing attention has been drawn to the potential diabetogenic effect of atypical antipsychotics. The goal of this prospective study is to evaluate the early effect of olanzapine and risperidone treatment on pancreatic beta-cell function in atypical-naive schizophrenic patients. Twenty-six subjects were assigned randomly to therapy with olanzapine or risperidone for 14 days. The metabolic parameters were quantitatively assessed by using the intravenous glucose tolerance test. The levels of fasting glucose, fasting insulin, lipid profiles, and leptin were also assessed. There were no significant within-group changes in weight or body mass index for both groups after 2 weeks of treatment. The levels of fasting glucose, fasting insulin, cholesterol, or leptin did not change in both groups. The triglyceride level significantly increased in olanzapine group. Glucose disappearance rate and insulin sensitivity did not change in both groups. Insulin secretion significantly decreased in olanzapine group. After 2 weeks of olanzapine treatment, schizophrenic patients decreased insulin secretory response to a hyperglycemic challenge. The results of this study support the hypothesis that olanzapine might directly impair pancreatic beta-cell function.