BACKGROUND: Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infection (LRTI) in infants throughout most of the world, but little is known about RSV infection in Africa where LRTI are among the leading causes of infant and childhood death. METHODS: The study took place in a rural district hospital in southern Mozambique between October, 1998, and May, 2000. From all children (n = 5635) <1 year of age presenting to an outpatient department with cough or nasal secretion and all children (n = 1307) <5 years of age admitted to hospital with a LRTI, a nasopharyngeal aspirate was collected and tested for RSV by enzyme-linked immunosorbent assay (Abbott). RESULTS: RSV infection was found in 8.6% of study infants in the outpatient department and 10.6% of admitted children with LRTI. Cases presented in predictable yearly outbreaks during the warm and rainy season. Lower respiratory tract involvement was frequent (59.7%). Cough, chest indrawing and increased respiratory rate were all independently related to RSV infection. Wheezing was infrequent. Bacterial coinfection (4.6%) and the case-fatality rate (3.4%) were low. CONCLUSIONS: There is a substantial burden of disease attributable to RSV infection in this rural African setting, with the highest incidence and severity occurring in young infants.