This prospective study examined the frequency and severity of respiratory illnesses in survivors of preterm birth compared with those in full-term infants. Although preterm infants did not demonstrate an inherent risk of subsequent respiratory illness when compared with full-term infants, earlier and more severe lower respiratory illnesses were observed among survivors of idiopathic respiratory distress syndrome (RDS). Infants who survived RDS but who developed residual lung disease had a greater risk of both more frequent and more severe subsequent lower respiratory illnesses than did RDS survivors who did not have persistent roentgenographic changes. We suggest that the risk of increased respiratory illness in these infants was a consequence of residual pulmonary abnormalities apparent on the chest roentgenogram at discharge from the nursery. Agents associated with respiratory illnesses were similar in all groups of study patients.