Asthma is a chronic condition that affects approximately 14 million persons in the United States and is characterized by airway inflammation, reversible airway obstruction, and airway hyperresponsiveness to a variety of triggers. Both environmental and psychological factors can trigger asthma exacerbations, and a seasonal increase in asthma morbidity occurs in the fall. This report summarizes the results of a telephone survey conducted among Manhattan residents 5-9 weeks following the September 11, 2001, terrorist attacks on the World Trade Center (WTC) in lower Manhattan in New York City. The findings indicate that among the 13% of adult respondents with asthma, 27% reported experiencing more severe asthma symptoms after September 11. Although a normal seasonal increase in asthma severity was expected, increased severity was reported more commonly among asthmatics reporting psychological distress associated with the attacks and/or difficulty breathing because of smoke and debris during the attacks. Persons with asthma and their clinicians should be aware of the role environmental and psychological factors might play in worsening asthma after disasters.