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 juvenile idiopathic arthritis (JIA). This evidence concerns the gene MIF and juvenile idiopathic arthritis.