We studied the appearance of IgE in the respiratory tract in 42 infants and young children with various forms of respiratory illness after infection by respiratory syncytial virus (RSV). IgE was bound to exfoliated nasopharyngeal epithelial cells in most patients with RSV infection during the acute phase of infection, regardless of the form of illness. However, the continued presence of cell-bound IgE was more common in patients with RSV-induced bronchiolitis or asthma than in patients with mild upper-respiratory-tract illness or pneumonia due to RSV. Persistence of IgE was also apparently related to the incidence of previous episodes of wheezing in patients or their families. The production of IgE and the subsequent release of chemical mediators of bronchospasm may contribute to the pathogenesis of acute illness due to RSV; persistence of IgE in the respiratory tract may explain the recurrent episodes of wheezing that occur in many patients after RSV-induced bronchiolitis.