Clinical studies revealed correlation of changes in anti-oxidative system and lipid peroxidation with peritonitis phases. The assessment of anti-oxidative system was used as an additional diagnostic and prognostic criterion for the disease. Data are presented in proof of efficiency of an ozone physiological solution included in combined treatment of acute peritonitis. Ozone therapy helped to normalize functioning of the anti-oxidative defensive system and thereby decrease the incidence of severe complications and fatal cases among patients with peritonitis.