Cystatin C, a low molecular weight protein, is a new endogenous marker of renal function whose serum concentration correlates better with glomerular filtration rate than creatinine. The aim of the present study was to define a reference interval for cystatin C concentrations in children. Cystatin C was measured by an immunoturbidimetric assay in sera obtained from 258 children (93 girls, 165 boys, median age 6.29 years, range 1 day to 18 years) without evidence of kidney disease. The reference interval was calculated non-parametrically using the 2.5th and 97.5th percentiles. For comparison, creatinine was measured in the same samples. The cystatin C concentration was highest on the first days of life (range 1.64-2.59 mg/l) with a rapid decrease during the first 4 months. Beyond the 1st year, the cystatin C concentration was constant, with a reference interval of 0.7-1.38 mg/l. In contrast, serum creatinine concentrations steadily increased with age until adulthood. Compared with creatinine, cystatin C facilitates the recognition of abnormal renal function in children as its reference range is constant beyond the 1st year of life. The higher levels of cystatin C in the 1st year of life probably reflect the low glomerular filtration rate of neonates and infants.