To assess the role of oxidative stress in exercise intolerance in patients with heart failure, a symptom-limited exercise test was performed with a bicycle ergometer in 12 patients with heart failure and in 7 members of a healthy control group. Level of plasma malondialdehyde, a marker of lipid peroxidation, at rest was significantly higher in patients than in the control group. The level further increased at peak exercise in patients but not in the control group. A significant inverse correlation was observed in patients between exercise-induced changes in malondialdehyde level and erythrocyte superoxide dismutase activity (r = -0.73, p < 0.01), and those exercise-induced changes in malondialdehyde level and superoxide dismutase activity had a close correlation with peak VO2 (r = -0.63, p < 0.05 and r = 0.67, p < 0.05, respectively), whereas no such correlation was observed in the control group. These findings suggest that the oxidative stress could be related to exercise intolerance in patients with heart failure.