Fasting insulin concentration and the insulin response to an oral glucose tolerance test were measured in 15 women with polycystic ovary syndrome (PCOS) and 6 weight-matched normal women. The fasting insulin concentrations were not increased in PCOS in comparison to normal women. Insulin response to an oral glucose test was significantly decreased (p less than 0.05) in PCOS at 30 min, although 63% of these women had mild impairment of glucose tolerance. The glucose response did not differ between the obese PCOS patients and those with nonobesity. No correlations were demonstrated between the fasting serum insulin concentrations and insulin response at 30 min, and both androgens and the luteinizing hormone. These findings suggest that (1) hyperinsulinemia is not a feature of PCOS in Japan; (2) only obese PCOS patients have a tendency of abnormalities in insulin secretion; (3) it is evident that PCOS represents an inadequately characterized disorder of insulin action and a novel paradigm in which to investigate the relationship between reproductive functions and carbohydrate metabolism.