BACKGROUND AND AIMS: Osteoarthritis (OA) is a chronic degenerative joint disorder of the synovial joint characterized by loss of articular cartilage, osteophyte formation, and alterations of subchondral bone. The aim of this study was to evaluate endoglin levels in both plasma and synovial fluid of patients with primary knee OA and to determine their relationship with disease severity. METHODS: Thirty nine patients with primary knee OA and 15 healthy controls were recruited in this study. The radiographic grading of OA in the knee was performed using the Kellgren and Lawrence classification. Endoglin concentrations in both plasma and synovial fluid were analyzed using a sandwich enzyme-linked immunosorbent assay. RESULTS: The average value of plasma endoglin in patients with knee OA was significantly higher compared with that of healthy controls (5.16+/-0.22 vs. 4.43+/-0.3 ng/mL, p=0.03). Although endoglin levels in synovial fluid were higher with respect to paired plasma samples, the difference was not significant (5.41+/-0.32 vs. 5.16+/-0.22 ng/mL, p=0.3). Additionally, plasma endoglin levels exhibited a positive correlation with synovial fluid endoglin levels (r=0.52, p=0.001). Further analysis revealed that plasma endoglin levels significantly correlated with disease severity (r=0.38, p=0.02). Furthermore, the synovial fluid levels of endoglin also positively correlated with disease severity (r=0.55, p=0.001). CONCLUSIONS: These findings indicate that endoglin in plasma and synovial fluid is correlated with progressive joint damage in knee OA. Endoglin is likely to be useful as a biomarker for determining disease severity and may play a possible role in the pathogenesis of osteoarthritis.