Osteoarthritis (OA) is a degenerative disorder that is characterized by articular cartilage destruction. The level of tenascin-C (TNC), an extracellular matrix protein, is increased in synovial fluid (SF) from OA patients, and TNC concentration in SF correlates with the severity of OA in the knee. Tenascin-X (TNX), another member of the tenascin family, has been identified as a causative protein of classical-like Ehlers-Danlos syndrome (clEDS). In this study, we investigated the correlation between TNX concentration and the degree of cartilage denaturation in OA patients. TNX concentrations in SF from patients with meniscus tears and from OA patients were lower than those of control sera. However, significant difference in TNX concentration in SF was not observed between patients with meniscus tears and OA patients. Furthermore, a significant correlation was not found between TNX concentration in SF and severity of OA in the knees of patients. These results suggest that TNX might not be as useful as TNC as a biomarker.