Objective To evaluate the role of polymorphonuclear (PMN) in patients with acute myocardial infarction (AMI).The preventive effects of Vitamin C-a potent free radical scavenger,on parameters of PMN oxygen free radicals (OFR) production in patients with AMI were also studied.Methods 60 patients with AMI were randomized to receive either conventional treatment only (group Ⅰ,n=30) or conventional treatment supplemented with vitamin C infusion,3.0 g/d,for 7 days (group Ⅱ,n=30).Parameters of PMN OFR production were assayed by a method named as polymorphonuclear chemiluminescence (PMN-CL) on the 1st,3rd,7th and 10th day after been hospitalized.62 healthy controls of similar age,sex were also studied.Results Parameters of PMN-CL increased significantly in patients with AMI compared with that in the healthy controls.Parameters of PMN-CL decreased significantly in group Ⅱ on the 3rd,7th and 10th day compared with that in group Ⅰ(P<0.05,0.01,0.001,respectively).ST segment score was significantly higher in group Ⅱ than that in group Ⅰ on the 3rd,7th and 10th day.Conclusion These results indicate that the PMN act as a potential contributor to extension of tissue injury induced by OFR.Supplementation with vitamin C may suppress PMN OFR production and could be beneficial in preventing myocardial necrosis.