Objective To investigate the role of lysyl oxidase (LOX) in synovitis and cartilage destruction by comparing the expression of LOX in synovial fluid and synovium of active rheumatoid arthritis (RA) and active osteoarthritis (OA).Methods LOX in the synovium was detected by immunohistochemistry from 14 patients with active RA,24 patients with active OA and 20 patients with knee injury (the control group).LOX in the synovial fluid was measured by enzyme linked immunosorbent assay (ELISA) from 14 patients with active RA and 24 patients with active OA.T-test was used for statistical analysis.Results The level of LOX expression in active RA synovium (0.012±0.007) was similar to that in active OA synovium (0.013±0.011,P＞0.05).But the expression of LOX in synovium of active RA and active OA was significantly higher than that in synovium of the control group (0.003±0.004,P＜0.01).The amount of LOX in the synovial fluid of active RA [(1.9±1.4) μg/ml] was significantly higher than that of active OA [(1.0±0.4) μg/ml,P＜0.05].Conclusion High expression of LOX in the synovial fluid and synovium of active RA and active OA suggest that LOX may be involved in chronic synovitis and cartilage destruction,and may be related with the extent of synovitis and cartilage destruction.