Viral infections are known to be associated with severe exacerbations of asthma in children. In contrast, there is limited data that viral infections evoke acute episodes of asthma that require emergency care in adults. To determine the role of viral infections in exacerbations of asthma in adults, we examined 33 patients who presented to the emergency room with 35 exacerbations of asthma between September 1990 and March 1991 for the presence of a viral infection. A nasal swab was obtained for virus isolation by culture and rapid antigen detection by fluorescent staining. In 16 patients, serum was collected at initial presentation and 3 to 4 weeks later for acute and convalescent viral antibody titers. All patients had acute episodes of asthma ascertained by medical history and physical examination. About 56% of the patients with asthma exacerbations had symptoms suggestive of viral illness. Rapid antigen detection and viral cultures for influenza A and B, parainfluenza-1, 2, 3, respiratory syncytial virus, adenovirus, and rhinovirus were negative on all patients. Likewise, in all 16 patients tested, acute and convalescent serologic studies did not show a significant rise in titer by complement fixation test. Thus, despite symptoms consistent with viral infection, viral pathogens could not be shown by current virologic techniques. This study suggests that viral infection may not be as prevalent a precipitate of asthma in adults requiring emergency room treatment as is generally thought.