Objective: to define the nosological category of juvenile arthritis (JA) in patients of different ages with different disease stages and follow-up periods. Subjects and methods. Group 1 included 135 children, mainly girls (60.7%), aged 1 to 16 years (range 7.9±5.0 years) with early-onset JA, the duration of which was 2 weeks to 6 months (range 2.9±1.6 months) who were further followed up for 2 years. Group 2 comprised 97 adult patients, mainly women (80.4%) aged 18 to 60 years (range 26.8±7.9 years) who had suffered from JA since childhood. The history of the disease varied from 10 to 53 years (range 20.5±8.2 years). The diagnosis in the children was made in accordance with the working criteria developed at the Pediatric Department of the Research Institute of Rheumatology, Russian Academy of Medical Sciences, for juvenile rheumatoid arthritis (JRA) and chronic JA (CJA). That in the adults was established by the diagnostic criteria for rheumatoid arthritis (RA), spondylarthritis (SA), ankylosing spondylitis (AS), and psoriatic arthritis (PA). Results. In the early-onset arthritis group, nonrheumatic diagnoses were made in 5 patients just in the first stage, which was a basis for their exclusion from the investigation. Reactive arthritis was diagnosed most frequently (46.3%) in the place of primary observation, the proportion of patients with this condition, when included into the study, was 4.4%; the diagnoses of JRA and CJA were established in 12.6 and 44.4% of cases, respectively. Later on, over 2 years of follow-up, the predominance (74.1%) of patients with CJA was retained. One patient with CJA developed obvious juvenile AS (JAS); JRA was verified in 3 children; acute rheumatic fever (ARF) and Kawasaki disease were in 2 cases; in 3 patients, rheumatic diagnoses (dysplastic coxarthrosis in 1 case, meniscopathy in 1, and leukoencephalitis in 1) were rejected. Ten years or more after disease onset, the adult patients with JA were diagnosed as having RA in 52% of cases; 11.5% had one of the types of SA; AS, PS, Crohn's disease- associated arthritis being in 4, 2, and 1 cases, respectively. Other diseases accounted for 11.5%. A diagnosis could not be verified in 25% of the patients; it was formulated as CJA or undifferentiated arthritis (UA). Among the patients with uveitis, the equal number (37.5%) RA or one of the types of SA; a diagnosis could not be specified in 25%, it was formulated as CJA (UA) or other diseases. Conclusion. This investigation has demonstrated that the diagnosis of JA presents certain difficulties and requires a thorough differentiated approach. Nosologically, the evolution of JAfollows the ways various joint diseases, such as JRA (RA), JAS (AS), other types of SA, develop; however, their diagnosis may be ultimately verified in different periods of the course of disease and a follow-up. The elaboration of current diagnostic criteria for JA, including those for JRA, will facilitate early disease recognition and timely selection of adequate and the most optimal patient management tactics