• Reduced serum COMP level may be a useful marker of active disease and growth retardation in juvenile idiopathic arthritis.
  
  
• Serum COMP might be a useful prognostic risk marker for severe growth impairment in recent-onset juvenile idiopathic arthritis.
  
  
• An increase in serum COMP might be a reliable biomarker to show that retarded growth is improved by treatments in systemic juvenile idiopathic arthritis.
  
  
• Serum COMP together with RFs and anti-CCP antibodies are important in the assessment of JIA patients to determine severity of the disease.