Thiopurines are immunosuppressive drugs used in the treatment of inflammatory bowel disease. They are metabolized into inactive metabolites, the most important of which is 6-methylmercaptopurine (6-MMP) and into the active cytotoxic metabolite 6-thioguanine nucleotide (6-TGN) which has an anti-inflammatory effect. The enzyme that catalyzes the formation of 6-MMP is thiopurine-S-methyltransferase (TMPT), susceptible to genetic polymorphisms that affect enzymatic activity. In addition to TPMT genetic polymorphisms, 6-MMP and 6-TG concentrations are influenced by pharmacokinetic and environmental factors. The aim of this study was to determine the concentration of thiopurine metabolites in the blood of patients with inflammatory bowel disease at steady state, to calculate the methylation index from the obtained concentrations, to determine the dependence of metabolite concentrations before and after dose adjustment, to evaluate metabolite concentrations according to therapeutic range and methylation index. Concentrations of thiopurine metabolites were determined by HPLC method. The study included 100 patients with inflammatory bowel disease who were on thiopurine therapy, aged 11 to a maximum of 65 years. Regression analysis showed that there is no linear dependence of thiopurine metabolite concentrations on the age of the subjects. It was concluded that age did not affect the values of metabolite concentrations in erythrocytes. In only 1/3 of patients, the expected concentrations of the active metabolite, 6-TG, were achieved. In order to optimize the dosing regimen and minimize side effects, therapeutic monitoring of thiopurine should be a routine method.