Introduction: Oxidative stress induced by the generation of excess reactive oxygen species is one of the cause of atherosclerosis finally leading to coronary heart disease (CHD). Glutathione reductase (GR), a flavoprotein antioxidant enzyme, regenerates glutathione (GSH) from its oxidized form, which is essential for scavenging of hydrogen peroxide by glutathione peroxidase. The aim of this study was to analyze the activity of GR and GSH level in serum of patients with various stages of CHD. Material and methods: Sixty consecutive patients with diagnosis of stable angina (SA, n=20), unstable angina (UA, n=20) and non-fatal myocardial infarction (MI, n=20) together with 20 healthy control subjects were enrolled. Venous blood samples were collected just before the coro-nary catheterization procedure (in study patients). GR activities and GSH concentration was deter-mined using commercially available colorimetric kits. Results: The highest GR activity and GSH serum level was noticed in UA patients. However, sta-tistical evaluation showed that only glutathione reductase activity was significantly higher in serum obtained from myocardial infarction patients, compared to stable angina pectoris patients (19.52 ± 13.88 vs 11.63 ± 5.45 nmol/min/ml for MI and SA, respectively; p<0.048). There were no signifi-cant differences in GSH between the CHD patients and controls. Conclusions: The elevated activity of glutathione reductase in serum of patients with unstable angi-na pectoris and myocardial infarction suggests the role of antioxidant system acute coronary syn-dromes.