To determine the frequency of viral, mycoplasmal and bacterial infections in adult asthmatic patients, 19 patients were examined for the presence of these organisms during wheezing exacerbations and on a routine monthly basis when the asthma was under control. Subjects were studied during consecutive periods that ranged from 8-19 mo. During 243 routine visits, 8 viral infections were identified (3.3%). Of 47 sputum specimens, 29 contained pathogenic bacteria (6%) but this frequency was due primarily to chronic colonization in 4 patients. There were 4 episodes of acute respiratory bacterial infection during routine visits (9%). During 84 wheezing exacerbations, 8 viral infections were found (11%), an incidence greater than that during routine visits (P < 0.02). Three acute bacterial infections were found during exacerbations (9%). Patients whose respiratory tracts were chronically colonized with pathogenic bacteria did not have an increased number of these episodes. Viral but not bacterial respiratory tract infections were significantly increased during wheezing exacerbations in adult asthmatic patients. Respiratory tract infections can occur without necessarily worsening the asthma. In some asthmatic patients the respiratory tract may be chronically colonized with pathogenic bacteria without increased frequency of asthmatic exacerbations.