To evaluate the respiratory effects of long-term exposure to air pollution, schoolchildren (ages 8 to 12 years) from two districts in Hong Kong with contrasting air quality were studied. Parents of 1660 children completed questionnaires on respiratory symptoms, and 1294 children had their ventilatory function tested with a spirometer. After adjustment for relevant covariates, children living in the more polluted district had increased odds ratios for frequent cough (1.74), frequent sputum (1.87), chronic sputum (1.84), and doctor-diagnosed asthma (1.98). Children of both sexes in the more polluted district had significantly poorer lung function, and the differences among girls were more marked. The study provides additional evidence for the adverse effects of long-term exposure to relatively low-level air pollution.