The relation between air pollution and the exacerbation of childhood asthma was studied in a panel of 71 children (aged 5 to 7 yr) with mild asthma who resided in the northern part of mexico City. During the follow-up, ambient measures of particulate matter less than 10 microns (PM10, 24-h average) and ozone (1-h maximum) frequently exceeded the Mexican standards for these contaminants. The peak expiratory flow rate (PEFR) was strongly associated with PM10 levels and marginally with ozone levels. Respiratory symptoms (coughing, phlegm production, wheezing, and difficulty breathing) were associated with both PM10 and ozone levels. An increase of 20 micrograms/m3 of PM10 was related to an 8% increase in lower respiratory illness (LRI) among children on the same day (95% confidence interval [CI] = 1.04-1.15), and an increase of 10 micrograms/m3 in the weekly mean of particulate matter less than 2.5 microns (PM2.5) was related to a 21% increase in LRI (95% CI = 1.08-1.35). A 50 parts per billion (ppb) increase in ozone was associated with a 9% increase in LRI (95% CI = 1.03-1.15) on the same day. We concluded that children with mild asthma are affected by the high ambient levels of particulate matter and ozone observed in the northern part of Mexico City.