Cystatin C is a non-glycated 13-kilodalton basic protein produced by all nucleated cells. The low molecular mass and the basic nature of cystatin C, in combination with its stable production rate, suggest that the glomerular filtration rate (GFR) is the major determinant of cystatin C concentration in the peripheral circulation. Recently published studies have shown that cystatin C correlates more strongly than creatinine with GFR measured using the 51Cr-EDTA clearance. The aim of this study was to evaluate serum cystatin C as a marker for GFR in children. GFR was determined on medical indications using the 51Cr-EDTA technique in pediatric patients (2-16 years) in our renal unit. Simultaneously their cystatin C and creatinine concentrations were also measured. Of our 52 patients, 19 had a reduced renal function (<GFR 89 ml/min per 1.73 m2) based on the 51Cr-EDTA clearance. The correlation of cystatin C with the isotopic measurement of GFR tended to be stronger (r=0.89, P=0.073) than that of creatinine (r=0.80). Receiver operating characteristic analysis showed that the diagnostic accuracy of cystatin C was better (P=0.037) than that of creatinine in discriminating between subjects with normal renal function and those with reduced GFR. This study demonstrates that serum cystatin C has an increased diagnostic accuracy for reduced GFR when compared with serum creatinine. Hence, cystatin C seems to be an attractive alternative for the estimation of GFR in children.