In a serologically based prospective study, acute infections with four atypical pathogens were determined in 100 adults hospitalized for acute exacerbation of bronchial asthma, and compared with the corresponding rate in a matched control group. Paired sera were tested using immunofluorescence or enzyme immunoassay methods to establish the serologic diagnosis. In 18 patients (18%), there was evidence of acute infection with Mycoplasma pneumoniae, compared with 3% in the control group (p = 0.0006). In 10 of these patients there was evidence of infection with at least one additional pathogen, a respiratory virus in 7. There was no significant difference between the study groups in the rates of acute infection by Chlamydia pneumoniae (8% in the hospitalized patients versus 6% in the control subjects), Legionella spp. (5 versus 3%, respectively), or Coxiella burnettii (no patients in either group). We conclude that of these four atypical pathogens, only infection with M. pneumoniae is associated with hospitalization for acute exacerbation of bronchial asthma. In most of these M. pneumoniae patients there is evidence of infection with a respiratory virus as well. The pathophysiologic and therapeutic significance of these findings should be tested in further studies specifically designed to address these questions.