Aim of the work: To measure interleukin-33 (IL-33) serum and synovial fluid (SF) levels as well as its relative expression in peripheral blood mononuclear cells (PBMC) of juvenile idiopathic arthritis (JIA) patients and to study their relation to clinical, laboratory and musculoskeletal ultrasound characteristics, disease activity and functional status. Patients and methods: The study included 60 JIA patients and 60 healthy controls and SF levels were measured in 20. Juvenile arthritis disease activity score (JADAS27) and Juvenile Arthritis Multidimensional Assessment Report (JAMAR) were assessed; Ten-joint grey scale (GS) and power Doppler (PD) MSUS score was performed. Rheumatoid factor (RF) titer and C-reactive protein (CRP) levels were measured. Results: In JIA patients, serum IL-33 levels (median 12.6; 7.4–23.8 ng/l) and its relative mRNA expression (median 3.3; 2.5–3.7) were significantly higher than their levels in the controls (median 1.7; 0.8–2.4 ng/l and median 1 ng/ml; p < 0.001). Polyarticular subtype (n = 20) had higher IL-33 serum levels compared to oligoarticular (n = 28, p < 0.001) and systemic-onset (n = 12, p = 0.006) subtypes. In JIA patients, the serum and SF levels of IL-33 significantly correlated with JADAS27 (p < 0.001 and 0.002 respectively), CRP (p < 0.001 and 0.007 respectively), GS (p < 0.001 and 0.001 respectively) and PD (p < 0.001 and 0.005 respectively). Serum IL-33 correlated with RF (p = 0.039) while, SF IL-33 correlated with physical function (p = 0.02). Conclusions: JIA patients have significantly elevated IL-33 serum concentrations and mRNA expression that considerably correlated with different inflammatory parameters, RF and physical function suggest