It is proposed that pulmonary defense mechanisms play an important role in stomach carcinogenesis. Inhaled carcinogens reach the stomach by normal pulmonary clearance and account for the association of stomach cancer with dusty occupations. We postulate that although familial and other factors determine the susceptibility to endothelial cancer (both stomach and lung), those persons whose pulmonary clearance mechanisms are more susceptible to impairment by cigarette smoke retain carcinogens and other particulate matter in the lungs and are at increased risk for lung cancer. Those whose clearance mechanisms are not impaired continue to clear these particles from their lungs, unconsciously swallow the cleared particles, receive more of these substances in the stomach, and are at increased risk for stomach cancer. This accounts for: first, the very striking dose-response relationship between the amount of smoking and lung cancer mortality and the relatively weak relationship between smoking and stomach cancer, and second, for observed reciprocal relationships in time trends of mortality between stomach cancer and lung cancer.