BACKGROUND: Glycated hemoglobin (HbA1c) levels represent a 2~3 month average of blood glucose concentration. The use of HbA1c as a diagnostic tool for diabetes is gaining interest. Therefore, we determined the cutoff point of HbA1c for predicting abnormal glucose tolerance status in non-diabetic Korean subjects. METHODS: We analyzed the data from 1,482 subjects without diabetes mellitus in whom a 75-g oral glucose tolerance test (OGTT) was performed due to suspected abnormal glucose tolerance. We obtained an HbA1c cutoff point for predicting diabetes using Receiver Operating Characteristic (ROC) curve analysis. RESULTS: A cut-off point of 5.95% HbA1c yielded sensitivity of 60.8% and specificity of 85.6%, respectively, for predicting diabetes. There was a difference in HbA1c cut-off value between men and women, 5.85% and 6.05%, respectively. CONCLUSION: To use the cut-off point of 5.95% HbA1c for predicting undiagnosed diabetes in Koreans may be reliable. However, studies of different ethnic groups have reported disparate HbA1c cut-off points. Thus, ethnicity, age, gender, and population prevalence of diabetes are important factors to consider in using elevated HbA1c value as a tool to diagnose diabetes.