Acute renal failure (ARF) is an acute loss of kidney function that occurs over days to weeks and results in an inability to appropriately excrete nitrogenous wastes and creatinine (Cre). ARF is diagnosed by elevations of blood urea nitrogen and serum Cre level, which is classified as prerenal, intrinsic and postrenal according to their mechanisms. However, discriminate diagnosis of these types by blood biochemistry findings is difficult. Recently, cystatin C (Cys-C), a basic protein having isoelectric point 9.3 with a molecular weight of 13.3 kDa, is freely filtered at the level of the glomerulus and virtually all is reabsorbed and metabolized by the proximal tubular cells. Therefore, assuming constant cellular production, serum Cys-C level has the potential to be an excellent surrogate marker of glomerular filtration rate. Because Cre is electrically charged neutrally, there is a possibility that the permeation of Cys-C, which is positively charged, is diffluent from that of Cre through glomerular basement membrane due to the type of the renal failure. We determined blood concentrations of Cys-C and Cre in a patients with prerenal renal failure (17 patients), intrinsic renal failure (232 patients) and postrenal renal failure (13 patients) as compared with healthy subjects (n = 771). We found that patients with postrenal renal failure displayed significantly elevated Cre/Cys-C ratio (mean +/- standard deviation) (8.3 +/- 8.0, p < 0.001) as compared with healthy subjects (1.1 +/- 0.2), prerenal (0.6 +/- 0.2) and intrinsic (1.6 +/- 0.5). These findings suggest that measurement of Cys-C concentration and Cre/Cys-C ratio may be useful for the discriminate diagnosis of postrenal renal failure.