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Urgency. Juvenile idiopathic arthritis (JIA) remains one of the most urgent issues of pediatric rheumatology due to incapacitation at early stages of the disease. It has been established that the first years of arthritis are key in the aspect of progression of the pathological process. Despite significant progress in treating JIA with genetically engineered biopharmaceuticals, some patients do not respond to such a therapy and thus aggravate the disease course. The study was aimed at improving diagnosis and treatment of children with JIA by means of analyzing the main immunological factors Th17, suggesting methods of differentiating lymphocytes and developing recommendations on assessing prognosis of the disease's course and outcomes. Methods. The study included 108 2–18-yearsold children with various forms of JIA. The control group was comprised of 18 conventionally healthy 6–17-years-old children with unburdened heredity regarding autoimmune diseases and without any clinical symptoms of diseases. The total T helper population and subpopulations thereof (naive Th cells and Th memory cells, double-positive Th cells), subpopulations of Th17 cells and anti-inflammatory cytokines (IL 1β, IL 6, IL 17A and TNF α) in the peripheral blood were analyzed in all the patients, including the control group. Results. The levels of Th17 memory cells and anti-inflammatory cytokines (IL 1β, IL 6 and IL 17A) were significantly higher in children with enthesitis-associated arthritis (HLA B27+ juvenile ankylosing spondyloarthritis) and systemic JIA. The level of Th17 cells and high blood level of IL 17A affect development of an active disease; simultaneous increase in IL 6 results in higher risk of osteochondral destruction in children with JIA. Conclusion. According to the immunological data, there are at least 2 JIA phenotypes: Th17-dependent and Th17-independent. Th17-dependent JIA is characterized by the most adverse course of the disease and high risk of osteoarticular destruction, which is why children require earlier prescription of genetically engineered biopharmaceuticals. By contrast, Th17-independent phenotype features a more favorable disease outcome and low risk of osteoarticular destruction.