Exposure to certain industrial agents has been thought to have carcinogenic potential, both for employees who work closely with such agents and for the general population that comes in contact with them. Although case reports, laboratory studies, and epidemiologic analyses help to determine the carcinogenicity of implicated agents, each of these types of investigation has limitations and deficiencies in distinguishing causal from noncausal associations. Asbestos has been linked with bronchogenic carcinoma, but several controversial factors—the degree of risk relative to exposure dose, the synergistic effect of cocarcinogens, and the question of existence of a threshold dose—complicate the understanding of the magnitude of the risk for exposed persons. Several other physical and chemical agents (such as chromium, nickel, and radon) have also been associated with an increased incidence of lung cancer in epidemiologic and animal studies. As with asbestos, the specific type of the agent and exposure conditions are important in determining the degree of carcinogenicity. In studies of exposure to man-made mineral fibers, formaldehyde, and silica, the findings have been inconsistent. Because the degree of health hazard attributable to asbestos and other known and suspected lung carcinogens is controversial, a wide range of opinions exists about the importance of occupational exposures to the overall incidence of lung cancers. Nevertheless, attempting to prevent lung cancers by minimizing or eliminating exposure to carcinogens is preferable to treating existent cases.