Osteoarthritis (OA) is primarily characterized by articular cartilage degradation. Hypoxia-inducible factor-1a (HIF-1a), a subunit of the basic helix-loop-helix-containing PER-ARNT-SIM (PAS) domain transcription factors, plays a vital role in the survival of articular chondrocytes to the hostile hypoxic microenvironment and complicates the progression of OA. In this study, we examined whether HIF-1a levels in the serum and synovial fluid (SF) of patients with knee OA were increased and whether the increase was correlated with the radiographic severity of the disease.