Epidemiologic studies over the last 40 years have observed that general ambient air pollution, chiefly due to the by- products of the incomplete combustion of fossil fuels, is associated with small relative increases in lung cancer. The evidence derives from studies of lung cancer trends, studies of occupational groups, comparisons of urban and rural populations, and case-control and cohort studies using diverse exposure metrics. Recent prospective cohort studies observed 30-50% increases in the risk of lung cancer in relation to approximately a doubling of respirable particle exposure. While these data reflect the effects of exposures in past decades, and despite some progress in reducing air pollution, large numbers of people in the US continue to be exposed to pollutant mixtures containing known or suspected carcinogens. These observations suggest that the most widely cited estimates of the proportional contribution of air pollution to lung cancer occurrence in the US, based largely on the results of animal experimentation, may be too low. It is important that better epidemiologic research be conducted to allow improved estimates of lung cancer risk from air pollution in the general population. The development and application of new epidemiologic methods, particularly the improved characterization of population-wide exposure to mixtures of air pollutants and the improved design of ecologic studies, could improve our ability to measure accurately the magnitude of excess cancer related to air pollution.