Aim: To study the relationship between the pathogenesis of lung cancer and antioxidant status and acidic media by measuring the activities of erythrocyte catalase (CAT) and carbonic anhydrase (CA). Methods: A total of 26 patients with lung cancer and 15 healthy individuals were included in the study. The CAT and CA activities of erythrocytes were defined. The catalase (CAT) activity of erythrocytes was measured using Aebi's method. Carbonic anhydrase (CA) activity was analyzed by CO2 hydration. Results: It was found that erythrocyte CA and CAT activities were significantly lower in patients with lung cancer compared to controls (p<0.05). Of the 26 patients with lung cancer, seven (26.9%) had metastasis, and the CA and CAT levels in patients with metastasis were significantly decreased (p=0,0001). Conclusions: Development of oxidative stress due to lung cancer may be related to the balance between prooxidant and antioxidant reactions. Catalase may have a preventive effect for malignant lung cancers and the gene of the antioxidant enzymes may be one of the anti-oncogenes, and inactivation of one of these genes in the process of carcinogenesis may lead to tumor development. This may be an explanation for the very low levels of antioxidant CAT in patients with lung cancer compared to healthy individuals. Carbonic anhydrase (CA) in tumor cells may be an indicator of the acid-base balance in lung cancer. Decreased levels of CA in patients with lung cancer may provide a convenient media for tumor development, growth and metastasis by creating an acidic media.