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We have investigated levels of antioxidant enzymes and free radical‐induced DNA base modifications in human cancerous lung tissues and in their cancer‐free surrounding tissues. Various DNA base lesions in chromatin of lung tissues were measured by gas chromatography‐mass spectrometry. Activities of superoxide dismutase, catalase and glutathione peroxidase were also measured in lung tissues. Higher levels of DNA lesions were observed in cancerous tissues than in cancer‐free surrounding tissues. Antioxidant enzyme levels were lower in cancerous tissues. The results indicate an association between decreased activities of antioxidant enzymes and increased levels of DNA lesions in cancerous tissues. Higher levels of DNA lesions suggest that free radical reactions may be increased in malignant tumor cells.