Introduction. Oxidative stress deserves special attention in the pathogenesis of sepsis. Material and methods. The study included 96 patients with abdominal sepsis caused by advanced suppurative peritonitis and destructive pancreatitis. All patients were divided into 3 groups depending on the severity of sepsis. Level of malondialdehyde (MDA) was determined to evaluate the intensity of lipid peroxidation (LPO). Proteins oxidative modification was assessed according to level of sulfhydryl groups (SH-groups) and carbonyls in proteins. State of anti-oxidant system (AOS) was defined based on activity of catalase, peroxidase, superoxide dismutase and glutathione peroxidase in erythrocytes. Results and discussion. There was no relationship between severity of multiple organ failure, MDA concentration, SH-groups and anti-oxidant enzymes levels. At the same time positive correlation between severity of multiple organ failure and carbonyls content in proteins was revealed. Conclusion. In surgical patients sepsis develops on background of oxidative stress. Significant reduction of SH-groups in proteins by the moment of sepsis diagnosis is an unfavorable factor for outcome. The degree of multiple organ failure in patients with abdominal sepsis correlates with oxidative injury of proteinic structures.