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The study aims at elucidating the leukocyte activation in the joint fluid of patients with acute traumatic hemarthrosis. Paired samples of peripheral blood and articular effusions after an acute hemorrhage were obtained from 22 patients. Leukocytes were separated and stained with fluorescein isothiocyanate (FITC)-conjugated mouse anti-human CD11a, CD18, and CD97 monoclonal antibodies for flow cytometry. The reactive oxygen species (ROS) production of leukocytes in corresponding samples of peripheral blood and joint effusion was measured via luminol dependent whole blood chemiluminometry. Significant decrease of CD11a density on monocytes, but markedly enhanced expression of CD97 and CD18 on polymorphonuclear neutrophil granulocytes (PMN), and significantly increased proportion of CD97 positive lymphocytes were found in the joint fluids as compared to the corresponding peripheral blood samples. Moreover, significantly decreased lag time and elevated rate of ROS production were revealed by chemiluminometry in case of joint derived leukocytes. Our results provide good evidence for intraarticular leukocyte activation during acute hemarthrosis. Since the activation precedes synovial inflammation, it is suggested that the leukocytes play an important role as an initiator in the pathogenesis of acute hemarthrosis., (Copyright 2006 Orthopaedic Research Society)