BACKGROUND: Previous studies have shown that progression from normal glucose tolerance(NGT) to impaired glucose tolerance(IGT) is associated with the development of insulin resistance and hyper-insulinemia, while further progression from IGT to NIDDM results from an inability of the 8-cell to maintain high rate of insulin secretion. However, it is not established whether similar findings are also observed in Korean subjects with glucose intolerance. The aim of this study was to examine insulin secretory response after oral glucose stimulation in obese and non-obese Korean subjects according to varying degree of glucose intolerance. METHODS: Eighty eight Korean men underwent 75g oral glucose tolerance test. The subjects were classified into NGT(n=30), IGT(n=23), NIDDM(n= 35) according to National Diabetes Data Group criteria. Obesity was defined as body mass index (BMI) > 25 kg/m . Serum true insulin and C-peptide concentrations were measured by radioimmunoassay. RESULTS: Fasting serum true insulin and C-peptide levels were not different from each other among NGT, IGT and NIDDM groups, both in obese and non-obese subjects. Obese subjects with IGT had significantly higher serum true insulin and C-peptide levels at 120 min than those in NGT subjects, but the levels at 30 and 60 min were not different. On the other hand, non-obese subjects with IGT had lower serum true insulin level at 30 min and lower serum C-pepitde level at 60 min compared to those in NGT subjects. True insulin and C-pepitde levels at 30 and 60 min were significantly lower in patients with NIDDM than in those with NGT, both in obese and non-obese subjects. CONCLUSION: Hyperinsulinemia, especially at a later phase of oral glucose tolerance test, is apparent in obese subjects with IGT. On the other hand, early phase insulin secretory defect is prominent in non-obese subjects with IGT. These results suggest that impaired insulin secretion may play a primary role in the pathogenesis of non-obese NIDDM in Korea.