 This is an epidemiologic study of the incidence of severe RSV LRTI among Brazilian children born 35WGA. The study will enroll competitively 350 subjects from 3 sites in Brazil. Preterm children will be identified after birth and up to 72 hours after hospital discharge and followed for one year after the enrollment. Subjects will be seen monthly for the first 6 months and bimonthly until one year of follow-up. At each visit, a medical and social history will be updated and a physical examination will be performed. Legal representative will notify study personnel in the event of a respiratory illness or hospital admission at which time the subject will be seen by an investigator. Children with LRTI diagnosed by physical examination will have viral diagnostic tests performed. Subjects with a LRTI not requiring hospitalization will be followed by telephone weekly until illness resolution. Hospitalized children with a LRTI will be followed daily while hospitalized and followed by telephone until illness resolution. All medical interventions and outcomes will be recorded. 