In occupational cancer epidemiology, many studies are carried out without access to information on smoking and other potential confounding variables. It is unclear whether such deficiencies are likely to cause serious bias in estimates of cancer-occupation associations. An empiric investigation was carried out to determine the effect of inclusion or exclusion of three variables--smoking, ethnic group, and socioeconomic status--on estimates of odds ratios (OR) between 25 occupations and three types of cancer--lung, bladder, and stomach. Of the 75 associations studied, only one OR was distorted by more than 40% when comparing unadjusted with adjusted estimates; three were distorted by between 30% and 40%; four others by between 20% and 30%. Of the eight associations which were distorted by more than 20%, seven involved lung cancer and one involved bladder cancer; none involved stomach cancer. An additional analysis was carried out on the 25 lung cancer-occupation associations to determine whether the nature of the stratification on smoking (ie, whether crude or "precise" categories were used) gave different OR estimates. The differences in ORs induced by different parametrizations of the smoking variable were relatively small. Our results support the view that relative risks between lung cancer and occupation in excess of 1.4 are unlikely to be artifacts due to uncontrolled confounding. For bladder and stomach cancer, the corresponding cut point may be as low as 1.2. In studies of occupation and cancer, uncontrolled confounding due to smoking and social class may not be as serious a threat to the integrity of results as is sometimes feared.