Cu, Zn-SOD values were measured by enzyme immunoassay in the synovial fluid, leukocytes in the synovial fluid, synovial membrane, and leukocytes in blood of the patients with rheumatoid arthritis. SOD activity, lipoperoxide value in serum, ESR, and Lansbury's index of the patients with rheumatoid arthritis under hyperbaric oxygen (HBO) therapy were also investigated. SOD values of synovial fluid and of leukocytes in synovial fluid from rheumatoid arthritis group were found to be higher than those from osteoarthritis group. No significant difference was found the SOD values in leukocytes of blood and synovial membrane between two groups. In the patients with rheumatoid arthritis under HBO therapy the SOD activity was increased, whereas lipoperoxide values was decreased. Furthermore, ESR and Lansbury's index showed a remarkable recovery. These results suggest that HBO therapy may be an effective treatment for the patients with rheumatoid arthritis.