A case-control study of the etiology of lung cancer in women was conducted in the Niagara Region of Ontario, because of local concerns about a high incidence of lung cancer. 51 female patients with lung cancer and 45 matched controls were interviewed. Information was collected about active and passive smoking, occupation and residential history. There was a strong association between active cigarette smoking and lung cancer (ever/never odds ratio 10.0; p less than .001) and 85% of the cases of lung cancer were attributed to active cigarette smoking. No other factors were significantly associated with lung cancer; there was weak evidence of an association between urban environment during childhood and lung cancer (p = 0.07). Associations between lung cancer and air pollution, and residential history, were not demonstrated, contrary to public perception. Thus, a previously reported excess of lung cancer in Niagara females is most likely attributable to cigarette smoking.