The study aimed to assess how many adult patients with juvenile idiopathic arthritis (JIA) treated with biologics fulfill classification criteria for adult rheumatic diseases and to evaluate the course of JIA in adulthood. 138 patients with JIA over 18 years old treated with biologics were included in a cross-sectional observative study. Among 138 adult patients with JIA treated with biologics, 81 patients remained with JIA diagnosis. 57 patients were rediagnosed. 31 patients met the criteria for spondyloarthropathy, among them 18 patients for ankylosing spondylitis, 10 patients for psoriatic arthritis, and 3 patients for non-radiographic axial spondyloarthritis. Rheumatoid arthritis was diagnosed in 24 patients and adults’ Still disease in 2 patients. 84 patients of all adults with JIA received one biologic agent, 40 received two biologic agents, and 14 received three or more biologic therapies. 10 patients received biologic agents out of recommendations for JIA. Of the adult JIA patients treated with biologics, 41% met the classification criteria for adult inflammatory diseases. Spondyloarthropathy and rheumatoid arthritis were most commonly diagnosed. Nearly 40% of adult JIA patients required at least one modification of biological treatment. Therefore, it is worth considering a revision of JIA to adult-onset inflammatory disease entities, as it broadens the spectrum of disease-modifying drugs.