It has been suggested that superoxide dismutase (SOD) plays an important role in endothelial dysfunction in essential hypertension (EH), by competing with nitric oxide for superoxide, thus influencing nitric oxide bioavailability. To answer the question of whether endothelial dysfunction is consequence of altered SOD expression we determined SOD activity in patients with different stages of EH. In this study 45 EH patients and 25 normotensive subjects were included. EH patients were divided into the three groups according to the guidelines of European Society of Hypertension. SOD activity was determined spectrophotometrically in RBC and plasma of EH patients and controls. The results obtained have shown that all groups of EH patients exhibit lower SOD activity than control normotensive subjects. Significant correlation between SOD activity and both diastolic (p<0.05, r=-0.394) and systolic blood pressure (p<0.05, r=-0.356) was found. Lowering of SOD activity in patients with different stages of EH leads to inefficient detoxification of superoxide in EH. An excess of superoxide of both cellular and extracellular origin takes part in enhanced degradation of nitric oxide and altered vasodilation, and consequent endothelial dysfunction.