This study was performed to investigate whether synovial proliferation (SP) differentially affects hypoxia in the joint cavities of rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Thirty RA and 42 OA patients who underwent synovitis assessment were classified into two groups based on the presence or absence of SP, as revealed by musculoskeletal ultrasonography. Synovial fluids (SFs) from the knee joints were analyzed for interleukin (IL)-8, pO(2), and white blood cell counts and blood samples were analyzed for erythrocyte sedimentation rate (ESR). No difference was found between the OA patients with and without SP in terms of SF oxygen tension (SF pO(2)) or IL-8 level, whereas the RA patients had significantly lower SF pO(2) levels in their knee joints than did the OA patients with SP, and the RA patients had higher levels of IL-8 in their joints than did the OA patients. The counts of infiltrated immune cells in the SF and tissues were much higher for patients with RA and SP than for those with OA and SP. The ESRs were not found to be correlated with SP in OA patients but were negatively correlated with SF pO(2) levels in RA patients. We conclude that ultrasonographically detected SP in OA patients does not generate a more hypoxic SF than that found in OA patients without SP. The SFs from RA patients with SP are hypoxic, which indicates that SP may have different impacts on hypoxia in the joint cavities of RA and OA patients.