The aim of the study was an assessment of various risk factors for nephrotoxicity of ifosfamide (IF) in children taking into account the importance of the concentrations of toxic metabolites of the drug excreted with urine and the polymorphism of genes encoding S-glutathione transferases of mi, pi, and theta classes (GSTM1, GSTP1 and GSTT1). The study was carried out in 37 children aged 2-17 years (mean age 8.9 +/- 4.5 years) treated with IF in 3 g/m2 dose for various malignant diseases. For the assessment of the incidence of deletion of GSTM1 and GSTT1 genes PCR method was applied while in the case of GSTP1 gene the polymorphism of A-G codon 105 was detected by the PCR-RFLP method. Before and after each treatment cycle the cumulative ifosfamide dose was calculated and the biochemical indices of renal canalicular and glomerular function were assessed which were graduated according to extended WHO criteria. Additionally, nuclear magnetic resonance 31P NMR method was applied for calculation of the concentrations of ifosfamide nephrotoxic metabolites and of the unchanged drug excreted with urine. The analysis performed demonstrated that in children with GSTP1 gene A-G codon 105 transition, a statistically significantly (p = 0.01) higher urinary excretion of toxic ifosfamide metabolites occurred, that increased with the cumulative drug dose. The age, sex and deletions of GSTM1 and GSTT1 genes exerted no effect on the concentrations of the toxic metabolites excreted with urine. The results of the studies demonstrate that GSTP1 gene mutations are the genetic risk factor for nephrotoxic complications of ifosfamide use.