OBJECTIVE: To describe the epidemiology of respiratory syncytial virus (RSV) infection in a developing country. METHODS: The work was carried out in three hospitals for primary cases and in the community for secondary cases in the western region of the Gambia, West Africa. RSV infection was diagnosed by immunofluorescence of nasopharyngeal aspirate samples in children younger than two years admitted to hospital with acute lower respiratory infection (ALRI). Routine records of all children with ALRI were analysed, and the incidence rates of ALRI, severe RSV-associated respiratory illness and hypoxaemic RSV infections were compared. A community-based study was undertaken to identify secondary cases and to obtain information about spread of the virus. FINDINGS: 4799 children with ALRI who were younger than two years and lived in the study area were admitted to the study hospitals: 421 had severe RSV-associated respiratory illness; 55 of these were hypoxaemic. Between 1994 and 1996, the observed incidence rate for ALRI in 100 children younger than one year living close to hospital was 9.6 cases per year; for severe RSV-associated respiratory illness 0.83; and for hypoxaemic RSV-associated respiratory illness 0.089. The proportion of all ALRI admissions due to RSV was 19%. Overall, 41% of children younger than five years in compounds in which cases lived and 42% in control compounds had evidence of RSV infection during the surveillance period. CONCLUSION: RSV is an important cause of ALRI leading to hospital admission in the Gambia. Morbidity is considerable and efforts at prevention are worthwhile.