In critically ill patients, mild to moderate reductions in glomerular filtration rate are not instantly followed by parallel changes in serum creatinine (SCr). The aim of this study was to identify a value of serum cystatin C (cys-C) level as a marker for monitoring renal function in critically ill pediatric patients. Creatinine clearance was used to estimate glomeruler filtration rate (eGFR). The correlation between the inverse of serum cys-C and eGFR (r = -0.70, p < 0.0001) was better than the correlation between the inverse of SCr and eGFR (r = -0.27, p = 0.008). Serum cys-C was found to be superior to SCr to predict renal impairment (area under the curve for cys-C, 0.932 and for SCr, 0.658). It can be concluded that cys-C is superior to SCr for the detection of renal impairment in critically ill children.