The blood serum concentrations of creatinine and the low molecular weight proteins cystatin C, beta 2-microglobulin and retinol-binding protein were measured in 106 patients whose glomerular filtration rates were assessed by Cr-ethylenediaminetetraacetate (EDTA)-clearance determinations. The reciprocals of the serum concentrations of creatinine, cystatin C and beta 2-microglobulin were closely correlated to the Cr-EDTA-clearance (r = 0.73, 0.75 and 0.70, respectively) in contrast to the corresponding values for retinol-binding protein (r = 0.39). The calculated values of the glomerular elimination rate for creatinine and cystatin C were normally distributed in contrast to those for beta 2-microglobulin. The calculated glomerular elimination rate of cystatin C was not correlated to age, sex, type of disorder or disease activity. The results demonstrate that the serum level of cystatin C is a better measure of the glomerular filtration rate than the serum level of beta 2-microglobulin.