Smoking habits of different occupational groups were studied in a sample of persons who participated in health screening examinations carried out by the mobile clinic of the Institute of Occupational Health (Helsinki, Finland). The sample consisted of 1 990 men and 1 044 women. The smoking habits of the study population as a whole conformed with those of the entire Finnish population. Distinct differences in smoking habits were found, however, in different occupational groups. The effect of smoking habits on tentative rate ratios of lung cancer in occupational groups were estimated with a procedure presented by Olav Axelson. The groups were selected from extreme ends of the smoking habit variable. Extreme estimates were taken to maximize the confounding effect of smoking. When occupational groups were contrasted with the general population, the confounding effect was found to be smaller than usually believed.