['Cartilage oligomeric matrix protein (COMP) is a component of articular cartilage whose serum levels show a strong correlation with radiographic osteoarthritis (OA) status. It has recently been found, however, that COMP is also produced in synovium. To assess the hypothesis that synovitis affects serum COMP levels in patients with knee OA, we collected sera for COMP simultaneous with a clinical examination for synovitis.', 'Sera were collected from OA patients who fulfilled the American College of Rheumatology criteria for knee OA. Radiographs were classified according to the grading system of Kellgren and Lawrence. Synovitis was diagnosed clinically by joint tenderness plus swelling and/or increased warmth over the joint. COMP levels in sera were measured by inhibition ELISA with monoclonal antibody (mAb) 17-C10.', 'Serum COMP levels were significantly correlated with age, synovitis and an interaction of synovitis and OA severity. Synovitis showed the strongest effect on COMP levels (R=0.1587, P< 0.01), in contrast to C-reactive protein, duration of OA and OA severity score which showed no significant effect on COMP levels. Individual signs of synovitis, namely, joint tenderness and warmth had a significant effect on serum COMP levels while swelling alone did not.'] Does serum cartilage oligomeric matrix protein reflect the presence of clinically diagnosed synovitis in patients with knee osteoarthritis?