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Background: Insulin resistance and insulin sec- retory dysfunction are considered as pathogenetic meehanisms leading to type 2 diabetes mellitus. In Korea, clinical features of type 2 diabetes are quite different from those of western countries. There are many non-obese patients and some even experienced considerable weight 1oss around the onset of diabetes mellitus. We investigated the insulin secretory function and in vivo insulin sensitivity in Korean patients with type 2 diabetes. Methods: 38 patients with type 2 diabetes mellitus (age; 47.3+-9.1 yrs) and 30 control subjects (age; 25.72.7 yrs) were included in this study. Type 2 diabetic subjects were further divided into obese (BMI >25, n=13) and non-obese (BMI<25, n=25) groups. Insulin secretory responses to the 75g aal gluxse loading and euglycemic hyperinsulinemic clamp test were performed on all subjects. Results: Type 2 diabetic subjects had significantly lower serum insulin levels at 30 min of OGTT, regardless of their obesity, compared to the control subjects. Mean glucose disposal rates (M-values) were decreased by 36% in non-obese type 2 diabetic subjects and 58% in obese type 2 diahetic subjects compared to the control subjects. But, about half (12/25) of non-obese type 2 diabetic subjects and 30% (4/13) of obese type 2 diabetic subjects had normal insulin sensitivity, defined by 95% confidence interval of control subjects. Insulin sensitivity index (M-value) was correlated with BMI, WHR, fasting insulin, and HDL-cholesterol concentrations in type 2 diabetic subjects. Conclusion: In Korean type 2 diabetic subjects, impairment of early-phase insulin secretion may be an universal finding, but insulin resistance is observed in about 60% of subjects. This result suggest that there is pathogenetic heterogeneity of type 2 diabetes rnellitus in Korea.