Background: New York City residents were exposed to a variety of inhaled substances after the collapse of the World Trade Center. Exposure to these substances might lead to an increase in asthma severity, with residential distance from Ground Zero predictive of the degree of change.Objective: We sought to assess the effect of the World Trade Center collapse on local pediatric asthmatic patients. Methods: We retrospectively reviewed the charts of 205 pediatric patients with established asthma from a clinic in lower Manhattan's Chinatown. Clinical data were obtained for the year before and the year after September 11, 2001. Measurements included numbers of visits, asthma medication prescriptions, oral corticosteroid prescriptions, weekly doses of rescue inhaler, and peak expiratory flow rates. Residential zip codes were used to compare the asthma severity of patients living within and beyond a 5-mile radius of Ground Zero.Results: After September 11, 2001, these children had more asthma-related clinic visits (P = .002) and received more prescriptions for asthma medications (P = .018). No significant differences in oral steroid or rescue inhaler use were noted. Those living within 5 miles had more clinic visits after September 11, 2001 (P = .013); the increase in clinic visits for patients living more than 5 miles from Ground Zero was not significant. Mean percent predicted peak expiratory flow rates decreased solely for those patients living within 5 miles of Ground Zero during the 3 months after September 11, 2001.Conclusions: Asthma severity worsened after September 11, 2001, in pediatric asthmatic patients living near Ground Zero. Residential proximity to Ground Zero was predictive of the degree of decrease in asthma health.