Determination of renal function in patients with childhood cancer by a simple and accurate test is crucial. The aim of this study is to determine the incidence of nephrotoxicity in patients with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML) and non-Hodgkin lymphoma (NHL) and evaluate the importance of serum cystatin-C for an early predictor of impairment of renal function. 27 patients (ages varied from 1.5 to 16 years, median age 10 year-old) were enrolled to this study. Serum urea, creatinine, cystatin-C concentrations and calculated creatinine clearances of the patients were determined during the treatment. Twenty (74%) of patients were male and seven (26%) were female. The frequencies of the diagnosis for ALL, AML and NHL were 13 (48.1%), 8 (29.6%) and 6 (22.3%), respectively. The mean values of cystatin C during the treatment were 0.77 +/- 0.3 mg/l, 0.86 +/- 0.5 mg/l and 0.81 +/- 0.3 mg/l, respectively. The values of urea, cystatin-C, and creatinine clearance did not differ statistically by duration of the treatment. We did not observe any renal impairment in our study group by the treatment. Cystatin-C has high values of sensitivity and specificity to predict the glomeruler filtration rate. Therefore cystatin C might be useful for determination glomeruler filtration rate in children with cancer; especially who have difficulties in collecting 24-hours urine sample.