BACKGROUND: Diabetic nephropathy is the most frequent complication in patients with diabetes mellitus (DM). In clinical practice, the glomerular filtration rate (GFR) is often estimated from serum creatinine. Recently, serum cystatin C has been suggested being a better parameter for diagnosis of impaired renal function. We evaluated serum cystatin C as a potential new marker of GFR in diabetes patients. METHODS: Serum cystatin C and serum creatinine (sCr) were measured in 73 DM patients to evaluate their usefulness in diabetic patients. DM patients were divided into three groups (whole DM patients, albuminuric patients, and DM patients with sCr<1 mg/dL). Serum cystatin C and sCr were compared with creatinine clearance (CCr). RESULTS: The overall correlation coefficient for the reciprocal of serum cystatin C was superior to that of the reciprocal of serum creatinine in all three patient groups. With CCr cut-off values of 60 mL/min and 80 mL/min, receiver operating characteristic (ROC) plotting demonstrated that serum cystatin C had a higher sensitivity and specificity for detecting decreased GFR than did serum creatinine in all three patient groups. CONCLUSIONS: These findings suggest that serum cystatin C is superior to serum creatinine as a marker of GFR measured by correction or mean ROC-plot AUC in diabetic patients; therefore, serum cystatin C could be used for the early detection of the impairment of renal function.