In chronic inflammations of rheumatoid arthritis (RA), polynuclear leukocytes, macro-phages, synovial cells and other inflammatory cells release reactive oxygen species (ROS) and cause tissue impairment, and are regarded as the responsible agents in this pathological condition. In the present study, the electron spin resonance (ESR) method was used to measure superoxide dismutase (SOD) in 76 knee synovial fluid samples from rheumatoid knee joints and to compare the results with those of osteoarthritic knee joints and postraumatic arthritis. There was no significant difference in the SOD activity between PA and OA patients knee joint fluid, which was higher than that in patients with posttraumatic arthritis. The SOD activity in RA knee joint fluid showed no correlation with serum CRP (C-reactive protein) or erythrocyte sedimentation rate, indices of inflammation. According to Larsen's Grading of rtheumatoid knee radiography, patients with moderate RA changes (grade Ⅲ or Ⅳ) show higher SOD activity than patients with early (grade Ⅰ or Ⅱ) and terminal RA (grade Ⅴ). Our findings suggest that the SOD actvity in RA knee joint fluid is a valid index of articular destruction.