Aim To evaluate the preventive effect of vitamin C ,a potent free radical scavenger, on the markers of polymorphonuclear leukocyte (PMN) oxygen free radical (OFR)production, lipid peroxidation and defenses against oxidative stress in patients with acute myocardial infarction (AMI). Methods 30 patients with AMI were randomized to receive either conventional treatment only (control group, n=13) or conventional treatment supplemented with vitamin C infusion (vitamin C group,n=17), 2 g per day, for 7 days. All measurements were performed before treatment, on the 3rd and 7th day after been hospitalized. The PMN OFR production was assayed by a convenient and sensitive method named as polymorphonuclear leukocyte chemiluminescence (PMN-CL). The plasma lipid peroxides concentration measured as malondialdehyde (MDA), activities of superoxide dismutase (SOD) and glutathione peroxidase (GSHPx) were estimated by chemical measurement. Meanwhile, serum CK activity and ECG were also monitored. Results Parameters of PMN-CL decreased significantly in vitamin C group on the 7th day compared with that in controls. Plasma MDA concentration decreased significantly on the 7th day and plasma activity of SOD increased more quickly on the 3rd and 7th day in vitamin C group than that in controls, however, it showed no salutary effects of vitamin C on CK and GSHPx activities in vitamin C group. Arrhythmia occurre less often in vitamin C group than in controls on the 7th day. Conclusions Supplementation with vitamin C may suppress PMN OFR production, decrease the levels of lipid peroxide, increase the plasma activity of antioxidative enzymatic system, and could be beneficial in preventing complications in such patients.