To determine whether the magnitude of the respiratory syncytial virus (RSV)-specific IgE response at the time of an episode of RSV bronchiolitis in infancy accurately predicts the development of subsequent wheezing episodes, we observed 38 infants prospectively from the time of an episode of infantile bronchiolitis through 48 months of age. Peak RSV-IgE titers were measured at the time of the bronchiolitis episode using an ELISA procedure. Notation was made of both the number of subsequent wheezing episodes reported by parents and the number documented by a physician. Subsequent wheezing was documented by a physician in 20% of infants who did not develop an RSV-IgE response at the time of the bronchiolitis episode and in 70% of those with the highest responses (P less than 0.025). These results suggest that the magnitude of the RSV-IgE response at the time of RSV bronchiolitis is a useful prognostic indicator for recurrent wheezing.