Immunosuppressant toxicity is a limiting factor for the efficacy and safety of long-term therapy. Whether it stems solely from drug exposure, remains unclear.Overall, 207 children and adolescents at the mean age of 11+/-4.4, with primary renal allograft were analyzed. Immunosuppression regimens included CsA or TAC, combined with AZA or MMF and steroids. Drug-specific toxicities were diagnosed by renal biopsy and/or clinical criteria. Genotyping for MDR1, CYP3A5, IL1B, IL1RN, IL-6, IL-10, MCP-1, TGFB1, CCR5, VEGF and TNF-alpha gene polymorphisms was performed with the use of PCR and PCR-RFLP techniques.Nephrotoxicity was seen in 38.5% of patients treated with CsA and 29.5% - with TAC, while gingival hypertrophy was observed in 28% of CsA patients. Myelotoxicity was found in 3% of AZA-treated and 6.4% of MMF-treated patients. No significant correlation was seen between the patient's age, gender, type of pre-transplantation dialysis, donor age, graft origin or cold ischemia time, and the occurrence of drug-related toxicity. For CNIs, the drug exposure and the duration of treatment did not prove of significance either. TAC associated nephrotoxicity correlated with the CCR5 gene polymorphism, as the wt/32 genotype was found in 21% of patients with no detected toxicity (p<0.041) and in none of the nephrotoxicity cases. The presence of this genotype was also associated with significantly better graft function at 1 year post-transplant (GFR 115.104+/-28.40 vs 86.434+/-29.96; p=0.022). An association was seen between the MMF-induced myelotoxicity and the TNF-alpha G(-308)A polymorphism (p<0.005), but the MMF exposure was higher in patients who developed toxicity.Genetic background should be regarded one of the risk factors for immunosuppressant related toxicity in renal transplantation.