BACKGROUND: The oral glucose tolerance test (OGTT) for detection of diabetes is difficult to perform in clinical settings. The aim of this study is to evaluate the performance of a more practical detection test, combined fasting plasma glucose (FPG) and glycosylated hemoglobin (HbA1c), as a predictor of diabetes mellitus (DM) in a Korean sample. METHODS: We examined 2,045 (M = 1,276, mean age = 47.8 +/- 9.0 yrs) medical check-up program participants between January 2002 to December 2003. FPG, HbA1c and a number of other biochemical tests were performed at baseline and four after years after initial screening. Patients who originally presented with diabetes were excluded. The characteristics of newly-diagnosed DM patients and non-diabetic patients were compared. RESULTS: The incidence of newly diagnosed diabetes was 1.6% (32/2,045) after four years of follow up. The subjects in the DM group were older, had higher levels of SBP, DBP, FPG, HbA1c, triglyceride, HDL cholesterol, GGT and LDH (P < 0.05). In multivariate logistic regression analysis, FPG (odds ratio [OR] 1.124) and HbA1c (OR 4.794) were significantly correlated with onset of diabetes (P < 0.05). The interaction parameter between FPG and HbA1c was more than 1.0, indicating that the two effects are synergistic. The predictive cut-off values of HbA1c and FPG were 5.35% (area under curve [AUC] = 0.944) and 102.5 mg/dL (AUC = 0.930), respectively. CONCLUSION: The combination of HbA1c above 5.35% and FPG above 102.5 mg/dL predicted the onset of diabetes in a Korean sample. These results suggest that the combination of FPG and HbA1c may be useful for predicting progression to type 2 diabetes in east Asians.