The risk for hospitalization with respiratory syncytial virus infection during the first year of life was about five per 1,000 live births per year for infants born to low-income families in Houston from 1975 to 1979. The risk varied depending upon the intensity of the epidemic for a given season, the month of birth of the infant, and the level of passively acquired maternal antibody at the time of birth. Over 80% of the children hospitalized were less than 6 months of age; thus, most were born during the six months preceding the peak of RS virus activity. The neutralizing antibody titers in cord sera of 68 infants with culture-proven infections before 6 months of age were significantly lower than those of 575 randomly selected cord samples of infants born during the same period. The level of antibody at the time of birth was directly correlated with age at the time of infection. In addition, infants with more severe illnesses had lower levels of antibody in serum collected near onset of illness than did infants with milder illnesses. These observations demonstrate protection against RS infection in early infancy that is correlated with the level of maternal antibody, but it is not known if this protection is mediated directly by the passively acquired antibody or by some other mechanism.