A case-control study on lung cancer and the habit of cigarette smoking was carried out in Barcelona (Spain). Cases were 103 women newly diagnosed with primary lung cancer in 10 hospitals from the study area. Histologic confirmation was given in 101 cases, of which 53 were adenocarcinoma, 19 squamous-cell carcinoma, 9 small-cell carcinoma and 20 other types. Two controls per case were selected, matched by age, residence and hospital. Compared with the never-smokers, the odds ratios (OR), with corresponding 95% confidence intervals (CI), were 1.61 (0.4 to 6.9) for ex-smokers and 3.61 (1.6 to 8.3) for current smokers. The risk of lung cancer showed a good dose-response relationship with duration of the habit, average number of cigarettes smoked daily and cumulative cigarette consumption. The risk of lung cancer increased by 62% for each 10 pack-years. Depth of inhalation also showed a remarkable effect, independently of the intensity of the habit. Although mortality and incidence rates of lung cancer among women in Spain are lower than in other developed countries, the risk of lung cancer is that which would be expected according to the pattern of the smoking habit in Spanish women.