Toxicologic pathologists need to understand the comparative oncology of lung tumors because lung cancer is a common and serious cancer in the human population. Lung cancer in humans is known to be caused by cigarette smoke and a number of other carcinogens in the environment. Animal studies are needed to elucidate possible interactions with other potential carcinogens in environmental or occupational settings. In addition, knowledge of dose-response relationships and potential synergistic effects are needed to minimize harmful effects. Understanding the pathogenesis of common lung tumors will also aid in the prevention, diagnosis and treatment of the disease. Toxicologic pathologists need to remember several important points about lung tumors. The lung cancer response varies among species. Important factors in this variation are the nature of the administered carcinogen, the tissue dose of the carcinogen, the mode of exposure, the sensitivity of the test animal species and the similarity to the human response. Studies of molecular changes are important new tools to understanding lung carcinogenesis. For example, the molecular changes in lung tumors of mice and humans have a number of similarities that may be important in evaluating the significance of compound-induced lung tumors in mice.