We studied the selenium (Se) concentration in whole blood and plasma, glutathione peroxidase (GSH-Px) activity in red blood cells and plasma, as well as both of these parameters in cancerous and tumor-free lung tissue of lung cancer patients. Blood samples were taken from 84 cancer patients and 61 healthy controls. Normal and neoplastic lung tissues were obtained from 57 patients at the time of surgery. Se concentrations in whole blood and plasma were lower by 23% (p < 0.001) in patients compared with controls. GSH-Px activity in red cells was lower by 20.2% (p < 0.004) and in plasma by 11.7% (p < 0.05) in patients than in the control group. On the other hand, the tumor Se level was higher by 66.6% (p < 0.0001) and GSH Px activity by 49.5% (p < 0.0001) than in adjacent tumor-free tissue. No differences in Se concentrations and GSH-Px activities were found between squamous cell carcinoma and adenocarcinoma nor among the clinical stages of the disease. In the whole blood and plasma of cancer patients significantly lower Se concentrations were found in smokers than in nonsmokers. Significantly lower Se concentrations were also found among cancer patients who were smokers compared with controls. These findings show that in the blood of cancer the antioxidant ability, as measured by Se and GSH-Px, is reduced significantly. The cause of increased Se and GSH-Px in the malignant part of the lung is not understood and requires further studies.