Thus, the aim of the study was to analyze the relationship of the immune response mediator genes polymorphic loci (TNFA rs1800629, LTA rs909253, IL1B rs16944, IL2-IL21 rs6822844, IL2RA rs2104286, IL6 rs1800795, IL10 rs1800872, MIF rs755622, CTLA4 rs3087243, NFKB1 rs28362491, PTPN22 rs2476601, PADI4 rs2240336) variants with the methotrexate efficacy in JIA. The gene discussed is MIF; the disease is juvenile idiopathic arthritis.