The aim of our study was to evaluate the influence of steroid therapy on serum cystatin C (cysC) concentrations in patients with acute lymphoblastic leukemia (ALL) and idiopthiás thrombocytopenias purpura (ITP). We studied 17 patients with ITP (girls: boys =5:12, mean age: 7.6 yrs, range between 1 to 17 years) and 18 patients with ALL (girls: boys =6:12, mean age: 6.3 yrs, range between 2 to 17 years). CysC and white blood cell count (WBC) in both group of patients were determined before and after 300 mg/m(2) cumulative dose of steroid therapy. Corticosteroids increased the level of cysC in both groups of patients, however significant increase was found only in ITP patients between pre- and posttreatment values (0.96 +/-0.27 mg/L vs. 1.16 +/- 0.3 mg/L, p =0,02). Pretreatment cysC concentrations were within the reference range in patients with ITP but not with ALL and were significantly higher in ALL patients, than in ITP patients (1.23 +/- 1,12 mg/L vs. 0.96 +/- 0.27 mg/L, p =0,02). Pretreatment WBC of ALL patients were significantly higher than of ITP patients (22.58 G/L, min. 3.5 G/L, max. 102.1 G/L vs. 7.46 G/L, min. 4.8 G/L, max. 12.3 G/L, p =0.03). We have found significant correlation between pretreatment cysC and WBC values in ALL patients (p = 0.04). Although the concentration of cysC may be slightly and reversibly influenced by corticosteroid treatment, cysC is sensitive to detect early and moderate deterioration of GFR in children with cancer.