We conducted a population-based case-control study in Stockholm during 1989-1995 to investigate the risk of lung cancer from exposure to environmental tobacco smoke. The study base consisted of persons above 30 years of age resident in Stockholm County who had never smoked regularly (that is, one cigarette or more daily for 1 year). Cases of lung cancer were identified at the three major county hospitals responsible for diagnosis and treatment of lung cancer. A total of 124 cases (35 men and 89 women) and 235 population controls (72 men and 163 women) participated. The never-smoking status was validated by interviews with next-of-kin. The relative risk associated with ever-cohabiting with a smoking spouse was 1.2 [95% confidence interval (CI) = 0.7-1.9]. Ever-exposure at work to environmental tobacco smoke carried a relative risk of 1.6 (95% CI = 0.9-2.9). Risks tended to be more elevated in high-exposure groups and with recent exposures. Both sources of environmental tobacco smoke seemed important, and considerable misclassification of total exposure occurred for each variable used separately, in particular for the less common spousal exposure. For those currently exposed to environmental tobacco smoke from the spouse, at work, or both, the relative risk was 2.6 (95% CI = 1.0-6.5). Our data imply that information from major sources of environmental tobacco smoke should be combined to avoid important misclassification and that timing of exposure should also be taken into consideration.