The authors examined the association of long-term exposure to ozone (03) and ambient particulate matter <10 microm in diameter (PM10) with pulmonary function by using cross-sectional data of 10,240 middle-aged adults who participated in the Atherosclerosis Risk in Communities study. Air-pollution data came from the US Environmental Protection Agency's Aerometric Information Retrieval System. After an adjustment for important covariates, the inverse associations with forced vital capacity or forced expiratory volume in 1 second were observed separately for 03 and PM10). These associations were stronger among smokers than among the nonsmokers; among users of respiratory medications than among nonusers; and among participants currently experiencing respiratory symptoms than among those without symptoms. However, an additional adjustment for the study center eliminated the PM10 association. Moreover, center-specific associations between PM10 and pulmonary function were not significant, possibly as a result of the homogeneity of within-center exposures.