When assessing the relationship between disease activity and the concentrations of the studied proteoglycans, positive correlations were found between JADAS-27 index values and decorin concentrations in the blood of children with JIA in all studied groups, i.e., before treatment, after treatment with disease-modifying antirheumatic drugs and after 2 years of treatment with the biologic drug etanercept. Here, DCN is linked to juvenile idiopathic arthritis.