Aims This study examines if dglucose, two-hour postprandial (2hPP) minus fasting glucose (F), predicts glycemic control better than F or 2hPPglucose. Methods F and 2hPPglucose, and renal function variables; BUN, serum creatinine (Scr), and estimated GFR (eGFR), were obtained from 56 insulin treated diabetic adults. 2hPP–F(d) was calculated. Variables were compared when 2hPPglucose was <200 ( n = 23) or >200 mg/dL ( n = 33). Correlation coefficients were calculated for F, 2hPP or 2hPP–F(d) renal function variables versus those for glucose. Results Variables differed significantly between F and 2hPP ( t-test, p < 0.05) for all patients and when 2hPPglucose was < or >200 mg/dL, except dBUN at <200 mg/dL. When F, 2hPP or 2hPP–F(d) variables between 2hPPglucose < and >200 mg/dL were compared, dScr was significant ( p = 0.0327). Correlation coefficients between dglucose and dScr or deGFR, were significant for all patients ( r = 0.420, p = 0.0013, and r = −0.434, p = 0.0008, respectively) and for 2hPPglucose >200 mg/dL ( r = 0.523, p = 0.0018 and r = −0.513, p = 0.0023, respectively) but not 2hPPglucose <200 mg/dL. When dglucose increased by 100 mg/dL, dScr increased by 0.08 and 0.11 mg/dL, and deGFR decreased by 2.73 and 3.73 mL/min for all patients and >200 mg/dL, respectively. Conclusions dGlucose better predicts renal function changes than F or 2hPPglucose. Postprandial hyperglycemia (<200 mg/dL) control is crucial for renal protection in diabetes.