Exposure of nonsmokers to environmental tobacco smoke (ETS) is widespread in European countries, the most serious exposures occurring at home and in the workplace. Epidemiological studies available up to 1986 have been reviewed by several international and national authorities, which agreed in concluding that ETS exposure is causally related to lung cancer. A number of epidemiological studies have been published since then, and have confirmed this association. The possibility of positive results due to bias has been envisaged; it seems, however, that such bias could not explain the whole excess of lung cancer. Few data are available on confounders, such as diet and previous history of lung disease, that might be responsible for the association; however, there is no evidence that they play an important role. Moreover, the biological plausibility of a causal association is supported by the similarity of the composition of ETS and active smoke. The causal association between ETS exposure and lung cancer now seems well-established; however, its public health impact is still debated. Estimates are available from the United States, Canada, Australia, New Zealand and England.