['To evaluate digital flexor tendon sheath (DFTS) synovial fluid cartilage oligomeric matrix protein (COMP) concentrations as a molecular marker for intrathecal pathology.', 'Case control study.', 'Horses (n=46) with DFTS tenosynovitis; 23 fresh cadaver horses.', 'DFTS synovial fluid samples were collected from clinical cases with noninfected DFTS tenosynovitis and from control DFTS. Clinical and surgical findings were recorded, and dissection of control limbs was performed to confirm the DFTS to be grossly normal. Synovial fluid COMP was quantified using a homologous competitive inhibition ELISA.', 'Abnormalities were identified tenoscopically: intrathecal tendon/ligament tearing was identified in 37 cases and 9 had other lesions. In control horses, synovial fluid COMP was higher in younger horses. Clinical cases with intrathecal tendon/ligament tearing had higher synovial fluid COMP than either clinical cases with other lesions, or controls. In horses ≥5 years old, the sensitivity and specificity of the assay was high for diagnosing intrathecal tendon/ligament tearing.'] Does increased cartilage oligomeric matrix protein concentrations in equine digital flexor tendon sheath synovial fluid predict intrathecal tendon damage?