The serum levels of cystatin C and creatinine were determined in a population comprising 69 children, 1-16 years old, and including children with both normal and reduced glomerular filtration rate (GFR) as determined by Cr-EDTA clearance measurement. The overall correlation between the reciprocal cystatin C concentration and GFR was significantly stronger (p < 0.05) than that between the reciprocal creatinine concentration and GFR and this was true also for the subpopulation of children with reduced GFR. Receiver-operating characteristic analysis also indicated superior diagnostic accuracy of serum cystatin C compared to that of serum creatinine for reduced GFR. The serum cystatin C reference values (mean +/- 1.96 SD) determined for children over one year of age was 0.63-1.33 mg/l, which is similar to that previously determined for adults. Serum cystatin C appears to be an attractive alternative to creatinine for estimation of GFR not only in adults, but also in children.