Although mycoplasmal airway infection frequently exacerbates bronchial asthma, the cause of the initial onset of asthma remains unclear at present. In this report, we describe a patient in whom a previous acute mycoplasmal respiratory infection led to an initial onset of bronchial asthma. One month after the onset of the illness, cough and wheezing appeared. Pulmonary function studies revealed an airway obstructive dysfunction. Oral administration of bronchodilators resulted in a marked improvement of the asthmatic symptoms. An airway hyperresponsiveness to methacholine was demonstrated even 2 yrs after the initial onset of the illness, and IgE antibody specific to Mycoplasma pneumoniae was detected in the serum by use of enzyme-linked immunosorbent assay. An immediate skin test for M. pneumoniae was positive in addition to multiple positive skin tests. A bronchial inhalation challenge test with M. pneumoniae antigen also yielded a positive result. We conclude that the effects of mycoplasmal respiratory infections on the airway are multifactorial and involve a complex interplay of airway inflammation and IgE-mediated hypersensitivity.