The therapeutic response to thiopurines may result in either severe toxic or inadequate effect based on the interindividual genetic variability. Same drug doses of various anticancer drugs cause considerable interindividual differences in the therapeutic response. Genetic factors have a major impact on effectiveness of several anticancer drugs such as mercaptopurine, 5-fluorouracil, platinum agents, and cyclophosphamide. Heredity related differences in interindividual response to thiopurine therapy represent perhaps the most compelling evidence of pharmacogenomics' usefulness in identification of patients in risk for adverse drug reactions. A number of variations in the gene for thiopurine methyltransferase (TPMT) have been associated with the low activity of this enzyme. Patients with intermediate and low activity of TPMT have a greater incidence of thiopurine toxicity. This minireview summarizes results of studies assessing the role of genetic polymorphisms in the gene encoding TPMT and their relationship to the toxicity of thiopurines.