PURPOSE: We wished to compare the clinical effectiveness of cystatin C (CyC) and serum creatinine (sCr) to assess renal function in renal transplantation patients. METHODS: We compared the clinical effectiveness of CyC with that of the sCr to estimate 24-hour urine creatinine clearance (CrCl) in 72 adult recipients who underwent renal transplantation from January 2001 to December 2008. We analyzed the data in terms of accuracy, bias, precision and sensitivity as a function of length of time posttransplantation and CrCl value. RESULTS: The patients were divided into four groups according to CrCl value <30, <60, <90, and > or =90 mL/min/1.73 m2. The corresponding Cr-based glomerular filtration rate (GFR) estimates had accuracies of 0.71, 0.906, 0.963, and 1.00 within 50% of the reference, with biases (mean percentage errors) of 4.7, 5.32, -5.79, -31.33 mL/min/1.73 m2, and precisions (mean absolute percentage errors) of 7.57, 10.03, 14.52, and 31.33 mL/min/1.73 m2, respectively. The CyC-based GFR estimates had accuracies of 0.35, 0.79, 0.93, and 0.67 within 50% of the reference, respectively, with biases of 15.03, 13.37, -5.58, and -34.79 mL/min/1.73 m2 and precisions of 15.03, 14.80, 17.91, and 34.79 mL/min/1.73 m2. The sensitivity for detecting GFR below 60 mL/min/1.73 m2 was higher for CyC (0.96, 1, and 0.95) than for Cr (0.77, 0.75, and 0.82). CONCLUSION: CyC is a more sensitive indicator of low GFR (CrCl <60 mL/min/1.73 m2) than sCr. However, CyC-based GFR estimates are restrictive data, and are neither accurate nor specific. Therefore, to evaluate renal function, we may need a revised CyC-based GFR formula and close monitoring of sCr.