['To evaluate the prognostic utility of serum COMP level measured with a new sandwich ELISA, by correlating COMP level with outcome measures of osteoarthritis (OA) progression.', "Patients (N=48) had symptomatic primary knee OA of Kellgren-Lawrence (K-L) grade I-III and met ACR criteria. These patients were evaluated prospectively as part of a double-blind drug trial of 3 years' duration and represented the placebo arm of the study. Serum COMP levels were measured by sandwich ELISA with monoclonal antibodies 16-F12 and 17-C10 at baseline and at study end and levels were correlated with changes in (1) joint space width (JSW), (2) K-L grade, (3) Lequesne, and (4) WOMAC indices, over 3 years.", 'The change in JSW over 3 years, summed for both knees, correlated positively with serum COMP level at baseline as well as at study end. Patients were sorted by level of progression based upon a change in K-L grade summed for both knees over 3 years; patients who progressed by two K-L grades were shown to have had significantly higher COMP levels at baseline as well as at study end. Baseline and study end COMP levels did not correlate with the change of Lequesne or WOMAC indices. Baseline COMP levels correlated strongly with end serum COMP levels.'] Do serum levels of cartilage oligomeric matrix protein ( COMP ) correlate with radiographic progression of knee osteoarthritis?