['Monitoring renal function is crucial in children undergoing chemotherapy. To date, a combination of routine serum creatinine (SCR) monitoring with occasional determination of creatinine clearance ratio (CCR) is widely used as clinical standard for this purpose. Both methods have their limitations regarding diagnostic value (SCR) or practicability (CCR), especially in young children. Diagnostic alternatives, such as glomerular filtration rate (GFR) estimation formulas have not been proved to be superior. The aim of the study was to evaluate whether serum cystatin C (CysC) may have a diagnostic impact on pediatric patients.', 'CysC, SCR, several GFR estimation formulas (Counahan-Barratt, Ghazali-Barratt, Schwartz, Shull, Traub), and CCR were studied in 80 pediatric cancer patients (age range: 0.17-17.9 years) during their chemotherapy. Special attention was given to children under the age of 3 in whom accurate urine collection for CCR is difficult.', 'All parameters correlated similarly well with CCR. Total accuracy was 66% and 67% for CysC and SCR, respectively. In very young children (<3 years), correlation with CCR was for CysC r = -0.74 with an area under the curve (AUC) of 0.646, and for SCR r = -0.27 with AUC = 0.594. Total accuracy was 60% for CysC, 50% for SCR.'] Is serum cystatin C a suitable marker for routine monitoring of renal function in pediatric cancer patients , especially of very young age?