The authors evaluated whether the effects of air pollution on asthma in children differed by the regional socioeconomic status (SES) associated with the area in which children resided. They analyzed data on air pollution, asthma-related hospitalization, weather conditions, and SES collected from 25 subregions in Seoul from January 1 to December 31, 2002. The National Health Insurance Cooperation provided data for children aged younger than 15 years who were living in Seoul and hospitalized for asthma-related health conditions (ICD-10 codes J45-J46). Exposure to interquartile range increase of airborne particles ≤ 10 μm in aerodynamic diameter, sulfur dioxide, and nitrogen dioxide in districts associated with a lower SES, the estimated percent increase of hospitalization for asthma among children was 31% (95% confidence interval [CI] = 14%-51%), 29% (95% CI = 8%-53%), and 29% (95% CI = 5%-58%), respectively. This suggests that not only biological sensitivity markers, but also the SES of subjects, should be considered as potentially confounding factors.