To assess sociodemographic differences in beliefs about the health effects of cigarette smoking and passive smoke exposure, we recently surveyed 2092 adults in St. Louis and Kansas City, Mo. The percentages of respondents who knew that smoking causes lung cancer, emphysema, and heart disease were 76.7, 74.1, and 67.2, respectively. After multivariate adjustment, knowledge about smoking's health effects was generally lower among women, older respondents, those of lower education level, and current smokers. Blacks were generally less likely to appreciate the health effects of active smoking, but were more likely to acknowledge the health effects of passive smoking.