This review examines the relationship between severe pulmonary disease caused by respiratory syncytial virus (RSV) infection in infancy and later development of asthma or reactive airway disease (RAD). RSV infection accounts for 70% or greater of all cases of infantile bronchiolitis and has been linked to subsequent asthma or RAD, either directly or through a shared common predisposition. Several studies suggest that RSV bronchiolitis is an important factor in the development of asthma and possibly atopy, although the association is lost by the age of 13 years. The mechanism is as yet unclear, but murine models of RSV disease have identified many plausible causal explanations. Further study is necessary to determine the relative roles of RSV infection and genetic predisposition in explaining the association between RSV infection and asthma/RAD.