Objective—Hyaluronic acid (HA) is an important joint marker and the substrate for hyaluronidase (HAase). Synovial fluid (SF) and serum HAase were measured to investigate the potential use of HAase as a joint marker in rheumatoid arthritis (RA) and osteoarthritis (OA) patients. Methods—The subjects were 39 patients with RA and 42 patients with OA. HAase activity was measured by zymography and its relation with various parameters examined statistically. Results—In RA SF a positive correlation (r=0.458, p=0.0186) was found between SF HAase activity and the concentration of serum C reactive protein. A positive correlation (r=0.45, p=0.024) was also found between SF HAase activity and platelet count in the RA group. Serum HAase activity in the RA group was significantly higher than in the OA group (p<0.0001) and normal controls (p<0.0001). Conclusion—The results suggest that SF HAase activity could be used as a marker of synovial inflammation. (Ann Rheum Dis 1999;58:186–188) Hyaluronic acid (HA) is a high molecular weight polysaccharide that is widely distributed in the body. The highest concentrations are found in connective tissue such as synovial membrane and synovial fluid (SF). There have been many reports on the use of HA as a joint marker, but there have been none investigating the use of HAase as such a marker. In 1984, Fiszer-Szafarz measured the HAase activity in serum from healthy human beings by zymography. In this study, we used zymography to measure HAase activity in serum and SF from rheumatoid arthritis (RA) and osteoarthritis (OA) patients, to examine the potential of HAase as a joint marker.