The relationship between respiratory infections and exacerbations of wheezing in patients with wheezy bronchitis or asthma has been described. Most respiratory infections were viral or Mycoplasma pneumoniae. This study was performed to evaluate the effect of Mycoplasma pneumoniae infection on bronchial reactivity and atopic background. 106 patients with Mycoplasma pneumoniae pneumonia who were admitted to the department of pediatrics, Fatima Hospital, Taegu during the period of two years from January 1989 to December 1990 were involved in this study. The results were as follows 1) Out of 106 cases, 69 were male,37 were female and 34.9% of the cases were between 5~6 years of age. 35 cases(33.0%) had wheezing and the incidence of wheezing were higher in male than in female. 2) The incidence of wheezing was high in the patients with serum IgE level higher than 200IU/ml, patients with past and family history of allergy and the patients fed with formula feeding in infancy. 3) Eosinophils and the associated maxillary sinusitis had no effect on the incidence of wheezing. 4) The recurrence of wheezing was higher in Mycoplasma pneumoniae pneumonia with wheezing(26.8%) than in patients without wheezing(3.8%).