BACKGROUND Estimated average glucose (eAG) is a value calculated from hemoglobin A1c (HbA1c) that reflects average glycemic status over the preceding few months. A linear relationship between HbA1c and eAG was demonstrated by the International HbA1c-Derived Average Glucose (ADAG) Trial in 2008. We investigated the relationship between fasting plasma glucose (FPG) and eAG. METHODS This retrospective study was conducted by reviewing the medical records of 6443 subjects, including 5567 diabetic patients and 876 non-diabetic subjects. The levels of HbA1c and FPG were reviewed and eAG was calculated using the regression equation published by the ADAG trial: eAGmmol/L=1.59×HbA1c(NGSP, %)-2.59[eAGmg/dL= 28.7×HbA1c(NGSP, %)-46.7]. RESULTS In all subjects, FPG showed a moderate correlation with eAG (r=0.672, p<0.001). When diabetic and non-diabetic subjects were divided into subgroups according to FPG level, the correlation between eAG and FPG decreased in both diabetic [FPG ≥10.0 mmol/L (180 mg/dL), r=0.425; FPG 7.2-9.9 mmol/L (130-179 mg/dL), r=0.373; FPG <7.2 mmol/L (130 mg/dL), r=0.202] and non-diabetic [FPG 5.6-6.9 mmol/L (100-125 mg/dL), r=0.363; FPG <5.6 mmol/L (100 mg/dL), r=0.186] subgroups as the FPG level decreased. The differences between eAG and FPG were statistically significant (p<0.001). Only 81% (4487/5567) of diabetic patients had a lower FPG level than eAG level. CONCLUSIONS Our results suggest that the relationship between eAG and FPG may depend on glycemic control, thereby enhancing our understanding of eAG.