BACKGROUND Bronchiolitis caused by respiratory syncytial virus (RSV) is an important cause of morbidity in ex-premature infants. In a randomised placebo controlled trial monoclonal antibody prophylaxis showed a 55% reduction in relative risk of hospital admission for these high risk infants, against a background incidence of 10.6 admissions per 100 high risk infants. AIMS To follow a cohort of high risk infants in order to assess hospitalisation rate from RSV and the potential impact of prophylaxis for these patients in a UK local health authority. METHODS A cohort of high risk infants from a local health authority were followed over the 1998/99 and 1999/2000 RSV seasons. The high risk population was defined as infants who, at the beginning of the seasons studied, were: (1) under 6 months old and born prior to 36 weeks gestation with no domiciliary oxygen requirement; or (2) under 24 months of age and discharged home in supplemental oxygen. All admissions with bronchiolitis during the season were identified. RESULTS A total of 370 high risk infants were identified for the 1998/99 season and 286 for the following year. Over the two years there were 68 admissions. Significantly more admissions occurred from group 2 infants. RSV was identified in 27 cases (four admissions per hundred high risk infants). Prophylaxis may have saved up to £195 134 in hospital costs over the two years, but would have cost £1.1 million in drug acquisition costs. CONCLUSIONS Careful consideration of risk factors is needed when selecting infants for RSV prophylaxis.