Leukocyte activation with cardiac surgery procedures produces various iuflammatory substances and involves in postoperative pathophysiology. The present study was carried out to elucidate changes in leukocyte myeloperoxidase level and effect on the heart and lung during cardiac operation. Total leukocyte and differential counts in peripheral blood, myeloperoxidase (MPO) and troponin-T concentratiens (TnT) in coronary sinus blood, and pulmonary vascular resistance (PVR) were measured at preoperative and postoperative period. The parameters were compared between sampling periods, and relationship was investigated between MPO and each variable. At the end of operation, there were leukocytosis with neutrophilia (p＜0.01), and increases of MPO and TnT concentrations (p＜0.05), but decrease in PVR (p＜0.05). MPO had a positive correlation to TnT, total leukocyte, neutrophil, or operative times (p＜0.05), whereas PVR had a negative relationship to total leukocyte or neutrophil counts (p＜0.05). These results indicate that cardiac surgery leads to elevated liberations of myeloperoxidase from neutrophils and may harmfully affect myocardium.