Objective. To determine the concentration of thiobarbituric acid reactive substances (TBARS) in erythrocytes and blood plasma, and the activities of selected antioxidant enzymes: catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) in erythrocytes in patients receiving hyperbaric oxygen (HBO) treatment due to difficult-to-heal “skin wounds”. Material and Methods. Indices of oxidative stress were assessed in venous blood taken from 23 patients three times: immediately before HBO procedure, approx. 5 minutes after leaving the hyperbaric chamber, and after 25 HBO procedures. Moreover, selected blood counts were measured in the collected material two times: prior to treatment and after 25 HBO procedures. Results. A statistically significant positive correlation between the CAT activity and the TBARS concentration in the erythrocytes of patients was found before treatment in the hyperbaric chamber (r=0.394; P≤0.05). No statistically significant changes in the TBARS concentration in erythrocytes and blood plasma were observed both after the first HBO procedure and after 25 procedures. No statistically significant changes in the activities of CAT, SOD, and GPx were noted. Platelet count decreased by 18.7% (P≤0.05) after 25 HBO procedures. Granulocyte count decreased by approx. 21% (P≤0.05), and granulocyte percentage by 11.8% (P≤0.01). In turn, the percentage of lymphocytes and monocytes increased after the treatment by 16.6% (P<0.05) and 16.4% (P<0.05), respectively. Conclusions. Exposure to HBO due to difficult-to-heal skin wounds does not significantly affect the levels of oxidative stress in the peripheral blood of patients and, from the point of view of oxidation–reduction processes, appears to be a safe therapeutic method for the treatment of chronic wounds.