The association between respiratory syncytial virus (RSV) infections in infancy and respiratory abnormalities later in life has been attributed both to a direct effect of the infection itself and to an inherent susceptibility. Observational studies do not allow a rigorous test of these hypotheses. Respiratory syncytial virus infection is universal in the first years of life and no uninfected control group exists. Randomized, controlled trials using new prophylactic agents such as vaccines or specific therapeutic agents will provide a powerful test of the relationship between RSV infection and long-term respiratory sequelae.