OBJECTIVE—Hyperglycemia increases glomerular filtration rate (GFR), but the influence of HbA1c (A1C) on GFR and GFR’s prediction by recommended equations remains to be determined. RESEARCH DESIGN AND METHODS—In 193 diabetic patients, we searched for an association between A1C and isotopically measured GFR (51Cr-EDTA) and their predictions by the Cockcroft and Gault formula (CG) and the modification of diet in renal disease (MDRD) equation. Their accuracy for the diagnosis of moderate (GFR <60 ml/min per 1.73 m2) or severe (GFR <30 ml/min per 1.73 m2) renal failure was compared from receiver operating characteristic (ROC) curves, before and after categorizing the patients as well (A1C ≤8%) or poorly controlled. RESULTS—The mean GFR was 57.0 ± 34.8 ml/min per 1.73 m2 and was well correlated with both estimations (CG r = 0.75, MDRD r = 0.83; P < 0.05). The areas under the ROC curves were higher with the MDRD (P < 0.05). A1C was correlated (P < 0.001) with the GFR (r = 0.29), MDRD (r = 0.38), CG (r = 0.26), and the absolute differences between the GFR and their CG but not MDRD estimations (r = 0.17, P < 0.05). Each +1% A1C was associated with +6.0 ml/min per 1.73 m2 GFR (CG +5.6, MDRD +5.3). After separating well-controlled (n = 88, A1C 7.0 ± 0.7%) and poorly controlled (n = 105, 9.6 ± 1.3%) patients, the diagnostic accuracies were better (P < 0.05) for the MDRD, except for the diagnosis of moderate renal failure in well-controlled patients (NS). CONCLUSIONS—GFR and its estimations correlate with A1C. Although the relations between GFR and its estimations were not affected by the degree of glucose control, the precision and diagnostic accuracy of the CG formula were diminished for A1C >8%. The MDRD equation was more accurate and robust in diabetic patients with impaired renal function.