Cystatin-C is a low-molecular-weight basic protein produced at a stable rate by all nucleated cells. It is freely filtered through the renal glomeruli and primarily catabolized in the proximal tubule cells. Since the serum cystatin-C concentration is not affected by muscle mass nor inflammation, it has been postulated to be an improved marker of glomerular filtration rate (GFR) compared with the serum creatinine level. To evaluate the clinical usefulness in terms of estimation of the glomerular filtration rate(GFR), we compared the serum cystatin-C concentration with other markers of GFR, such as serum levels of creatinine(SCr), alpha 1-microglobulin(alpha 1MG), and beta 2-microglobulin(beta 2MG). Their variations were analyzed based on 2-hour creatinine clearance (2hCCr) as a standard marker of GFR. The logarithmic value of serum cystatin-C level showed a stronger negative correlation(-0.959) with the logarithmic value of 2hCCr than that of other markers(-0.924, -0.942, -0.888; SCr, alpha 1MG, beta 2MG, respectively). Although beta 2MG showed the next strongest correlation with 2hCCr, it had a significantly lower sensitivity when detecting mild reduction of GFR. In addition, serum cystatin-C showed the greatest area under the curve of receiver-operating characteristic(ROC) of all GFR markers at both higher(90 ml/min.) and lower(70 ml/min.) cut-off value of 2hCCr. These data suggest that serum cystatin-C is useful for estimating GFR, even if the reduction of GFR is very mild.