Specimens from 49 persons, 3 to 60 years of age, who were prone to attacks of apparently infectious asthma were cultured for viruses and bacteria during episodes of symptomatic respiratory infection from September 1972 to June 1973. Seventy-one of 128 episodes (55 per cent) of symptomatic respiratory infection occurred with asthma. Shedding of respiratory pathogens was associated with 43 of the 128 episodes, and 19 of the 43 etiologically defined episodes occurred with wheezing. Seven of 15 rhinovirus symptomatic respiratory infections were linked with asthma, as were 4 of 5 influenza type A (H3N2) and 2 of 3 respiratory syncytial virus infections. A total of 21 different rhinovirus serotypes have been identified from asthmatic patients during 2 years of study; 14 of these were associated with wheezing, indicating that the ability to cause asthma is not restricted to only a few types. Other viruses that were shed during wheezing-associated symptomatic respiratory infections included para-influenza type 3, adenovirus type 7, and 2 unidentified viruses. Although infectious asthma was not easily defined in adults and some older children, viruses that appeared to precipitate asthma were distributed among patients of widely differing ages.