Objective: To determine protein and activity levels of matrix metalloproteinases 1 and 3 (MMP-1 and MMP-3) in synovial fluid of patients with knee joint injury, primary osteoarthritis, and acute pyrophosphate arthritis (pseudogout). Methods: Measurements were done on knee synovial fluid obtained in a cross sectional study of cases of injury (n=283), osteoarthritis (n=105), and pseudogout (n=65), and in healthy controls (n=35). Activity of MMP-1 and MMP-3 in α2 macroglobulin complexes was measured using specific low molecular weight fluorogenic substrates. ProMMP-1, proMMP-3, and TIMP-1 (tissue inhibitor of metalloproteinase 1) were quantified by immunoassay. Results: Mean levels of proMMP-1, proMMP-3, and TIMP-1 were increased in injury, osteoarthritis, and pseudogout compared with controls. MMP-1 activity was increased in pseudogout and injury groups over control levels, whereas MMP-3 activity was increased only in the pseudogout group. The increase in MMP-1 activity coincided with a decrease in TIMP-1 levels in the injury group. Conclusions: Patients with joint injury have a persistent increase in proMMP-1 and proMMP-3 in synovial fluid and an increase in activated MMPs, which are not inhibited by TIMP. The differences in activation and inhibition patterns between the study groups are consistent with disease specific patterns of MMP activation and/or inhibition in joint pathology. Chemicals / CAS: alpha 2 macroglobulin, 95568-41-5; interstitial collagenase, 9001-12-1; stromelysin, 79955-99-0; tissue inhibitor of metalloproteinase 3, 145809-21-8, 164781-40-2; alpha-Macroglobulins; Matrix Metalloproteinases, EC 3.4.24.-; Tissue Inhibitor of Metalloproteinase-1