We measured the content of glutathione and activity of glutathione-metabolizing and antioxidant enzymes superoxide dismutase and catalase in samples obtained from 52 patients with malignant lung tumors and 20 patients with benign lung tumors. The content of glutathione and activity of glutathione-metabolizing enzymes underwent similar changes, but these changes were most pronounced in malignant tumors. Antioxidant enzyme activity changed insignificantly in benign tumors, but significantly decreased in malignant tumors (squamous cell carcinoma and adenocarcinoma). The severity of changes in malignant tumors depended on the degree of malignancy. Most pronounced changes were observed in adenocarcinoma, which often metastasizes and is resistant to chemotherapy. These changes were least pronounced in bronchoalveolar carcinoma sensitive to chemotherapy.