The acute respiratory effects of a wintertime air pollution episode were studied in a general population sample of 112 children who were 7-12 y of age and who lived in a nonurban community. Spirometry was performed on 6 d, with a fixed interval of 3 wk between successive tests. During an air pollution episode, an additional pulmonary function test was made. Acute respiratory symptoms of the children were noted in a diary. Ambient concentrations of sulfur dioxide, black smoke, particulate matter with an aerodynamic diameter less than 10 microns, and nitrogen dioxide were considered as exposure variables. The association of air pollution with pulmonary function and prevalence of acute respiratory symptoms was assessed by individual linear regression analysis and time series analysis, respectively. In February 1991, an air pollution episode occurred during which daily average sulfur dioxide concentrations were slightly above 100 micrograms/m3, and particulate matter (with an aerodynamic diameter of less than 10 microns) concentrations reached 174 micrograms/m3. During the episode, forced vital capacity, forced expiratory volume in 1 s, and maximal mid-expiratory flow were lower than on baseline tests. Significant negative associations were found between the concentration of sulfur dioxide, black smoke, and particulate matter with an aerodynamic diameter of less than 10 microns. No association between prevalence of acute respiratory symptoms and the concentration of these compounds was found.