Creatinine clearance (Ccr) is generally used as a halmark of glomerular filtration rate (GFR) in clinical medicine. Recently, it has been suggested that serum cystatin C measurement in serum reflects GFR. We compared serum cystatin C, serum creatinine, and serum beta2-microgrobrin in 70 patients with renal diseases in reference to creatinine clearance. The correlation between Ccr and 1/cystatin C was higher (r = 0.830) than that between Ccr and 1/serum creatinine (r = 0.789) or 1/serum beta2-microgroburin (r = 0.732). The levels of serum cystatin C in patients with Ccr ranging from 51 to 70 ml/min were significantly higher than those in patients with Ccr ranging more than 91 ml/min. Receiver-operated characteristic (ROC) analysis revealed that serum cystatin C showed the highest area under the curve among the three when Ccr = 90 ml/min was used as the cutoff point. We conclude that serum cystatin C is more useful than serum creatinine to detect early renal dysfunction.