Lung cancer incidence and mortality have increased sharply during this century, making it a common cause of death and the most frequent fatal cancer in men and women. The known behavioral and environmental causes--cigarette smoking, asbestos and other occupational carcinogens, radon, and environmental tobacco smoke--are responsible for the majority of cases. Although the proportion of adults who smoke in the United States has dropped since the mid-1960s, the incidence of lung cancer in the United States is expected to continue to rise through the 1990s because of the long latency between initiation of smoking and the occurrence of the disease. Efforts to alter lung cancer mortality by early detection and better therapy have not yet succeeded in reversing this trend. The modification of risk factors is currently the most effective approach to reducing future lung cancer deaths.