Cartilage oligomeric matrix protein (COMP) is a noncollagenous extracellular matrix protein found predominantly in cartilage, but also in tendon, ligament and meniscus. Studies in man have demonstrated that it may be used as a prognostic marker in rheumatoid arthritis and osteoarthritis. The present study investigated whether tendon injury contributes to serum and tendon sheath synovial fluid levels of COMP in horses. COMP levels, analysed by competitive ELISA, in the digital sheath synovial fluid were more than 10-fold higherthan in the serum. Levels were significantly raised when tendon damage or sepsis was present within the tendon sheath but showed only mild, statistically insignificant, elevation in cases of tenosynovitis alone. COMP concentrations in serum were found to vary with age. Foals (age ≥ 1 year) had significantly (P<0.001) higher levels in comparison to older control horses. Total COMP concentrations in an age-matched group with tendinitis were not significantly different from the control group. Measurements of COMP levels in tendon sheath synovial fluid are therefore useful in depicting processes in tendon tissue, while elevated serum levels are likely to be more representative of joint disease than tendinitis.