The state of peroxide oxidation of lipids (POL) in the erythrocytes before and after the operation was studied. In patients with ulcer disease, the correlation between POL activity and changes in erythrocytic properties was noted. At the period of disease exacerbation, activation of POL processes in erythrocytes, suppression of the antioxidation protection (AOP), impairement in hemorheologic indices (increase in aggregation of erythrocytes, their resistance to acid hemolysis, reduction in their capacity for deformation) were revealed. Pronouncement of the pathologic changes after the operation depended on a background state of POL and AOP. In ulcer disease, the impairment in POL/AOP balance caused the development of postoperative complications. To prevent the development of complications, the administration of antioxidants is expedient.