OBJECTIVE Serum creatinine levels are lower in diabetic patients compared with their nondiabetic counterparts. Therefore, estimated glomerular filtration rate (eGFR) is higher in the former than in the latter group. Factors associated with overestimation of renal function in diabetic patients were examined, and new formulae reflecting precise eGFR were created. RESEARCH DESIGN AND METHODS Eighty subjects (age 56.5 ± 15.4 years; 35 males [43.8%]; 40 patients with diabetes and 40 nondiabetic subjects) were enrolled. GFR was evaluated by inulin clearance (Cin). eGFR values were calculated based on serum creatinine and/or serum cystatin C levels. The factors related to the dissociation between eGFR and Cin in diabetic patients and the agreement among each of three eGFR and Cin were compared. RESULTS Although Cin was not significantly different between the diabetic and nondiabetic subjects (P = 0.2866), each of three eGFR measures from the diabetic patients was significantly higher than that of the nondiabetic subjects (P < 0.01). There were significant and positive correlations between the ratio of each eGFR/Cin, hemoglobin A1c, and glycated albumin. The intraclass correlation coefficients in diabetic patients were weaker than those in the nondiabetic subjects, and the intercepts of the regression lines between each eGFR measure and Cin in the diabetic patients were significantly higher than those of the nondiabetic subjects. New formulae for the calculation of eGFR corrected by the glycemic control indices were better than the original eGFR, particularly in diabetic patients. CONCLUSIONS eGFR overestimates Cin as glycemic controls worsen. eGFR corrected by hemoglobin A1c is considered to be clinically useful and feasible.