Previous controlled studies have indicated that asthma medication modifies the adverse effects of sulfur dioxide (SO2) on lung function and asthma symptoms. The present report analyzed the role of medication use in a panel study of children with mild asthma. Children from Sokolov (n = 82) recorded daily peak expiratory flow (PEF) measurements, symptoms, and medication use in a diary. Linear and logistic regression analyses estimated the impact of concentrations of sulfate particles with diameters less than 2.5 microns, adjusting for linear trend, mean temperature, weekend (versus weekday), and prevalence of fever in the sample. Fifty-one children took no asthma medication, and only 31 were current medication users. Most children were treated with theophylline; only nine used sprays containing beta-agonist. For the nonmedicated children, weak associations between a 5-day mean of sulfates and respiratory symptoms were observed. Medicated children, in contrast, increased their beta-agonist use in direct association with an increase in 5-day mean of sulfates, but medication use did not prevent decreases in PEF and increases in the prevalence of cough attributable to particulate air pollution. Medication use was not a confounder but attenuated the associations between particulate air pollution and health outcomes.