Background: Whether glycosylated hemoglobin (Hb) A1c has any clinical significance in metabolic syndrome (MS) in nondiabetic subjects remains debatable. The aim of this study was to compare the diagnostic accuracy of MS using HbA1c versus the fasting blood glucose (FBG) in nondiabetic subjects. Methods: Study subjects were 1,474 Koreans in a rural area in Daegu city. They were enrolled from August to November 2008 for a cohort study. Of these subjects, 1,150 non-diabetic individuals with a HbA1c <6.5% (438 males; mean age=62.7±11.0 year-old) were finally analyzed in this study. Results: The area under the receiver operating characteristic curve of HbA1c for identifying subjects with the MS according to the FBG criterion was 0.634 (sensitivity 60.9%, specificity 59.4%), and optimum cut-off value was 5.75%. The HbA1c at range of ≥5.7% to 6.4% was used in the definition of the glycemic component of the MS. An 87.3% agreement existed between the use of HbA1c and the FBG for diagnosis of the MS (kappa coefficient=0.733); however, the proportion of subjects who met the MS criteria using the HbA1c was greater (43.0% versus 33.0%). The study subjects were divided into 4 groups based on the HbA1c levels, as follows: quartile 1 (<5.6%, n=283); quartile 2 (5.6% - 5.69%, n=162); quartile 3 (5.7% - 5.89%, n=304); and quartile 4 (≥5.9%, n=401). The frequency of MS increased as HbA1c level increased (20.5%, 25.9%, 32.6%, and 44.9%, respectively; p<0.001). Compared to the subjects who met the MS criteria using the FBG criterion, those with the MS using the HbA1c criterion alone had lower blood pressure, lower visceral adiposity, lower levels of inflammatory markers and liver enzymes, and lipid profile. The correlations of waist circumference (r=0.134, p<0.001), triglycerides (r=0.156, p<0.001), and the systolic blood pressure (r=0.184, p<0.001) with FBG were stronger than the corresponding correlations with HbA1c. Conclusions: Although a good agreement existed between the HbA1c and FBG criteria for the diagnosis of the MS, the diagnosis of the MS using the HbA1c have a possibility of “over-diagnosis” of MS.