Epidemiologic evidence on the relation between environmental tobacco smoke and cancer is reviewed. The labeling of tobacco smoke as an environmental cause of lung cancer has been challenged based on allegations of bias in the epidemiologic data. However, tobacco smoke has been shown to increase the risk of lung cancer down to the lowest exposure levels. Environmental tobacco smoke contains the same carcinogenic compounds as those found in the tobacco smoke inhaled directly by the smoker. Nonsmokers environmentally exposed have elevated levels of tobacco smoke byproducts in biological samples. These observations alone are sufficient to identify tobacco smoke as an environmental carcinogen. The epidemiologic studies showing that environmental exposure to tobacco smoke is associated weakly but consistently with increased risk of lung cancer. While these epidemiologic studies have been challenged, it does not appear that the observed epidemiologic associations are due to misclassification or confounding. Indeed, the epidemiologic results, particularly among the studies with superior data collection methods and better control of bias and confounding, find consistent associations between environmental tobacco smoke and lung cancer. This paper summarizes the evidence that environmental exposure to tobacco smoke increases the risk of lung cancer, and considers the criticisms of the epidemiologic evidence which have been raised.