Objective.Cartilage oligomeric matrix protein (COMP) has been identified as a prognostic marker of progressive joint destruction in rheumatoid arthritis. In this population based study we evaluated associations between plasma concentrations of COMP, disease activity, and growth velocity in patients with recent-onset juvenile idiopathic arthritis (JIA). COMP levels in JIA and healthy children were compared with those in healthy adults. Plasma levels of insulin-like growth factor I (IGF-1), which has been associated with COMP expression and growth velocity, were studied in parallel.Methods.87 patients with JIA entered the study, including oligoarticular JIA (n = 34), enthesitis-related arthritis (n = 8), polyarticular rheumatoid factor (RF)-positive JIA (n = 2), polyarticular RF-negative JIA (n = 27), systemic JIA (n = 6), and undifferentiated JIA (n = 10). Plasma levels of COMP were measured by ELISA and IGF-1 by a radioimmunoassay.Results.Significantly higher COMP levels [mean 18.9 U/l (95% CI 17.3–20.5)] were found in healthy children compared with healthy adults [mean 10.7 U/l (95% CI 9.4–12.1)] (p < 0.0001). COMP levels in the JIA patients [mean 13.5 U/l (95% CI 12.4–14.7)] were significantly reduced compared to healthy children (p < 0.0001), and correlated negatively with C-reactive protein (CRP; r = −0.29, p = 0.01) and thrombocyte count (r = −0.28, p = 0.02). COMP levels in the JIA patients correlated positively with growth velocity (cm/yr) (r = 0.38, p = 0.0003) and growth velocity (SDS) (r = 0.29, p = 0.007).Conclusion.We found reduced COMP levels in children with JIA compared with healthy children. COMP levels in JIA correlated negatively with inflammatory activity as evaluated by CRP and the thrombocyte counts, and were associated with reduced growth rate.