Infliximab is an anti-inflammatory drug or a chimeric monoclonal antibody of the IgG1κ subclass, which binds with high affinity to the soluble and membrane-bound form of tumor necrosis factor α and thus prevents TNF-α from binding to its receptor and its anti-inflammatory effect. It belongs to a group of biologic drugs. It is used in patients with chronic inflammatory bowel diseases, skin and rheumatic diseases. Targeting therapeutic drug concentrations are of 3-7 μg / mL. The purpose of the study is to determine the concentrations of infliximab in patients with chronic inflammatory diseases, depending on the time of the therapy start and the degree of correlation between the dose of and the concentration of infliximab in relation to the patient's clinical response to therapy. 23 patients were included in this prospective study. The 42 serum samples were collected from the 23 patients (for 19 patients samples were collected before dosing). The mean infliximab concentration was 4.6 (1.1 to 12.2) μg / mL. No correlation was found between the drug dose and the drug concentration (r = -0.14; p = 0.40), while negative correlation was found between the drug concentration and the week from the therapy start (r = -0.57; p <0.001). For patients in the initial phase of treatment (0-14 weeks) the concentration of infliximab was significantly higher (14.2 (6.5 to 20.1) μg / mL) than in patients who achieved state of remission (after 14th week) (2.47 (0.9 to 6.9) μg / mL; p <0.001). In conclusion , for reliable interpretation of therapeutic concentrations of infliximab is useful to determine the level after the initial phase , ie after 14th weeks when it reaches the equilibrium concentration of the drug.