BACKGROUND The secretory dysfunction of pancreatic B-cell is one of the important in the pathogenesis of NIDDM. And the conversion from IGT to DM is developed by the exhaustion and decompensation of 0-cell. So our purp was estimating the cut-off point of fasting blood sugar predicting B-cell decompensation during OGTT. METHOD: The clinical characteristics and anthropometric parameters were determined in all subjects. 75 g ora] glucose tolerance tests were performed with serial blood sampling to measure plasma glucose levels, insulin and C-peptide levels. And we calcolated insulin response areas and C-peptide response areas. RESULTS: l) The basal C-peptide levels were elevated in IG1' and DM group. however, post-load 30min C-peptide and 30min insulin levels were significantly decreased in DM group compared with normal and IGl. 2) IGT group showed the highest C-peptide response areas among three groups. 3) The relationships between fasting plasma glucose, C-peptide & insulin levels showed that basal C-peptide level had turning point at 6.7 mmol/L of fastiing glucose, basal insulin level at 6.5 mmol/L, 30 min C-peptide at 6.2 mmol/1, 30 min insulin at 5.8 mmol/L and C-peptide response area at 7.0 mmol/L. Conclusion : Above result suggest that the fasting plasma glucose of 7.0 mmol/L may be the cut-off point of pancreatic B-cell decompensation.