Aim We sought to evaluate the markers of polymorphonuclear (PMN) oxygen free radical (OFR) production, the levels of lipid peroxidation (malonaldialdehyde, MDA) and defenses against oxidative stress in the patients with acute myocardial infarction (AMI). Methods and Results A convenient and sensitive method for PMN OFR production was assayed by polymorphonuclear chemiluminescence (PMN-CL). We have measured the parameters of PMN-CL, the plasma MDA concentration, the plasma activities of antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPX) in 60 AMI patients and 62 age-matched healthy subjects. Patients with AMI showed significantly increased PMN-CL parameters as peak value, integration and phagocytic index compared to healthy controls (P<0.001,respectively). The plasma MDA concentration increased in AMI patients compared with that in healthy controls (P<0.001). The PMN-CL peak value correlated best with plasma MDA concentration(r=0.58,P<0.01,n=122) and negatively correlated with the plasma activity of SOD (r=-0.63,P<0.01, n=122). Conclusion The PMN act as a potential contributor to extension of myocardial necrosis induced by OFR, thereby indicating the involvement of free radicals in AMI.