Concentrations of interleukin (IL)-6 and IL-8 in serum and synovial fluid obtained from patients with osteoarthritis (OA) of the knee were determined by the chemiluminescence-ELISA (CL-ELISA) method, the sensitivity of which is 100-1,000 times greater than that of the conventional ELISA method. The results were compared with those obtained from patients with rheumatoid arthritis (RA) and from healthy subjects. The mean IL-6 and IL-8 levels in synovial fluid indicated higher concentrations in RA than in OA. The IL-6 and IL-8 levels in serum were significantly higher in RA and OA relative to controls. Among OA patients in whom remarkable improvement was noted in hydrarthrosis, the synovial fluid IL-6 and IL-8 levels at the initial examination were relatively higher, and were markedly decreased after treatment with sodium hyaluronate (NaHA). Among those in whom no improvement was noted in hydrarthrosis, the synovial fluid IL-6 and IL-8 levels at the time of initial examination were relatively lower, and hydrarthrosis was not significantly improved even after treatment with NaHA. In addition, there was a tendency for the synovial fluid IL-6 and IL-8 levels to decrease as HA levels increased. Evaluation of X-ray findings revealed that the IL-6 levels in synovial fluid at the initial examination in low-grade cases tended to be significantly higher than in high-grade cases. In low-grade cases, as determined by X-ray findings, there was a significant decrease in IL-6 levels in synovial fluid after treatment with NaHA.