The prevalence of respiratory symptoms, as ascertained by questionnaire, was evaluated in 6,666 nonsmokers who had lived for at least 11 years in either a high photochemical pollution area (4,379 individuals) or a low photochemical pollution area (2,287 individuals). Of these, 5,178 had never smoked, and none was currently smoking. The risk estimate for "definite" COPD, as defined in this study, was 15 percent higher in the high pollution area (p = 0.03), after adjusting for sex, age, race, education, occupational exposure, and past smoking history. Past smokers had a risk estimate 22 percent higher than never smokers (p = 0.01). Multivariate analysis showed a significant effect of air pollution on the prevalence of "definite" COPD which univariate analysis failed to demonstrate.