As part of a study of health effects of sulfur dioxide and particulate matter, the authors have established a cohort of adults 25 to 74 yr of age in 6 communities who will be followed prospectively. At the conclusion of the first cycle of measuring the health of adults in 6 sites it was found that, although different sampling frames were used, samples were close to the distribution shown in the U.S. Census for age, sex, and occupation, with the possible exception of one city. Analysis of the cross-sectional data indicated that for both age and height-adjusted values for forced expiratory volume in 1 s and for selected rates of various respiratory symptoms standardized for age, differences among smoking groups were apparent. Differences in these parameters between sites suggest trends that were associated with levels of pollution. Further analyses of the prospective data currently being collected will be required before definitive statements can be made about specific levels of exposure.