The possible role of viral infections in the inception of asthma has been the matter of considerable debate. Older data suggested that viral respiratory infections occurring during early life could alter the lungs and the immune system, thus starting the process leading to allergic sensitization and persistent bronchial responsiveness. More recent studies suggest that infants who wheeze belong to two distinct groups, which at present can only be differentiated by the evolution of their illness. Most of these infants have a transitory tendency to wheeze during viral infections, and their lung function shortly after birth is significantly lower than that of infants who will not wheeze during similar infections. Most of these children become symptom-free during the preschool years, and their condition is not associated with higher serum IgE levels. A smaller group of children who wheeze as infants will still have wheezing episodes during the early school years. The factors that determine which infants will become persistent wheezers are not well understood, but viral infections per se are likely to play a minor role, if any. Early sensitization to aeroallergens in subjects genetically predisposed to having high serum IgE levels seems to be the main risk factor for this condition.