BACKGROUND: This study aimed to investigate the bone morphogenetic protein-2 (BMP-2) levels in serum and synovial fluid (SF) of patients with primary knee osteoarthritis (OA) and to exam its correlation with radiographic and symptomatic severity of the disease. MATERIAL/METHODS: A total of 37 knee OA patients and 20 healthy controls were enrolled in this study. Knee OA radiographic grading was performed according to the Kellgren-Lawrence (KL) grading system by evaluating X-ray changes observed in anteroposterior knee radiography. Symptomatic severity of the disease was evaluated according to the Western Ontario McMaster University Osteoarthritis Index (WOMAC) scores. BMP-2 levels in serum and SF were determined using enzyme-linked immunosorbent assay. RESULTS: Serum BMP-2 level in patients with knee OA was higher than that in healthy controls. Knee OA patients with KL grade 4 showed significantly elevated BMP-2 levels in the serum and SF compared with those with KL grade 2 and 3. Knee OA patients with KL grade 3 had significant higher SF levels of BMP-2 than those with KL grade 2. BMP-2 levels in the serum and SF of knee OA patients were both positively correlated with KL grades and WOMAC scores. CONCLUSIONS: BMP2 levels in serum and SF were closely related to the radiographic and symptomatic severity of knee OA and may serve as an alternative biochemical parameter to determine disease severity of primary knee OA.