In 1946, when the causes of lung cancer were much less well understood than they are now, a meeting was held by the British Medical Research Council to review hypotheses to explain the remarkable increase in the death rates from lung cancer and to determine strategy. Stocks came away from the meeting to study the community aspects of air pollution, which he did by extending his series of correlation studies, Kennaway to conduct studies of carcinogens in the air, and Hill to carry out a study of smoking in relation to lung cancer. It is now known, of course, that cigarette smoking is by far the most important cause of lung cancer and that about a dozen occupational exposures are also established as causes of this disease. There has been continuing uncertainty about the role of general air pollution. During the past few years, this uncertainty has been compounded with anxiety that the increasing use of diesel-powered vehicles might lead to a deterioration in air quality and, with it, an increase in the incidence of lung cancer. The purpose of this paper is to assess the current role of air pollution as a factor in lung cancer and specifically the contribution of diesel exhaust emissions to the incidence of that disease.