To investigate the effects of 12 monthly average air pollution levels on monthly prevalence of respiratory morbidity, the authors examined retrospective questionnaire data on 2034 4th-grade children from 12 Southern California communities that were enrolled in The Children's Health Study. Wheezing during the spring and summer months was associated with community levels of airborne particulate matter with a diameter < or = 10 microm (PM10) (odds ratio (OR) = 2.91; 95% confidence interval (CI) = 1.46-5.80), but was not associated with community levels of ozone, nitrogen dioxide, PM2.5 (diameter < or = 2.5), nitric acid, or formic acid. Logistic regression was performed on data stratified into two seasonal groups, spring/summer and fall/winter. Among asthmatics, the monthly prevalence of asthma medication use was associated with monthly levels of ozone, nitric acid, and acetic acid (OR = 1.80 [95%CI = 1.19-2.70]; OR = 1.80 [95%CI = 1.23-2.65]; OR = 1.57 [95% CI = 1.11-2.21]; respectively). Asthma medication use was more prevalent among children who spent more time outdoors--with consequential exposure to ozone--than among children who spent more time indoors (OR = 3.07 [95%CI = 1.61-5.86]; OR = 1.31 [95%CI = 0.47-2.71]; respectively). The authors concluded that monthly variations in some ambient air pollutants were associated with monthly respiratory morbidity among school children.