Reperfusion of ischemic myocardium evokes rapid release of free radicals in experimental models. The aim of the study was to investigate the oxidative stress and antioxidative defense during first minutes after reopening of the infarct related artery in patients treated for acute myocardial infarction. The study group consisted of 15 patients with first ST elevation myocardial infarction (STEMI) due to left anterior descending artery occlusion. The control group included ten patients with stable ischemic heart disease (IHD). Blood samples from coronary sinus were drawn before, immediately after and about 15 min after angioplasty. Activity of superoxide dysmutase (SOD), concentration of glutathione as well as the concentrations of lipid peroxides, malodialdehyde (MDA) and 4-hydroxy-2-nonenal (HNE) were measured. There was significantly higher concentration of MDA and HNE and higher SOD activity in STEMI patients before the reperfusion, as compared to the stable IHD group. After the reperfusion concentration of HNE in erythrocytes from STEMI patients was higher than in IHD group. At the same time the activity of SOD significantly decreased in patients with impaired tissue perfusion (myocardial blush grade <2). In conclusion, there is a slightly higher concentration of oxidative stress parameters in patients with STEMI. Diminished antioxidative defense after reperfusion is associated with impaired myocardial perfusion.