Several data justify the suspension that air pollution may contribute to cancer risk. These observations are: high risk of lung cancer in coal gas workers; higher incidence of lung cancer in urbanized regions as compared with rural ones and higher incidence in highly industrialized countries as compared with less industrialized; observations in migrant populations; data of comparative pathology. Specific studies of the association between lung cancer and air pollution taking into consideration confounding variables failed to give conclusive evidence of a causal relationship. Obviously the role of air pollution for cancerogenesis in the lung is far less important than inhalation of tobacco smoke. Furthermore there are some findings suggesting a causal association of particulate air pollution and malignant neoplasms of the stomach and prostate.