Juvenile idiopathic arthritis is the most common form of arthritis in children and adolescents. Methotrexate is the mainstay of juvenile idiopathic arthritis treatment. Methotrexate is well known to cause serum aminotransferase elevations, and longterm therapy has been linked to the development of fatty liver disease, fibrosis and even cirrhosis. The aim of the study was to assess non-invasive liver fibrosis indexes in adolescents with juvenile idiopathic arthritis treated with methotrexate. Materials and methods: A total of 68 children with juvenile idiopathic arthritis were enrolled in the study. A total of 68 children – 25 boys (36.8%) and 43 girls (63.2%) – were examined. The children were divided into four groups based on the methotrexate cumulative dose. The following data were analysed: total bilirubin and its fractions, cholesterol, β-lipoproteins, aspartate transaminase, alanine aminotransferase. Fibrosis indexes, i.e. APRI, FIB-4 score, AAR, AARPRI, were studied. Results: When studying the dynamics of non-invasive indices, their heterogeneous nature and cumulative dose dependence are determined. Statistically significant changes in fibrosis indexes and biochemical parameters were observed when the children reached cumulative methotrexate doses of 1 g and 3 g. Conclusions: It is possible to monitor carefully liver condition during the whole treatment and conduct a more in-depth examination only if it is necessary. Our data shows that the APRI index (AST to platelet ratio index) is most indicative.