OBJECTIVES: To estimate the hospitalisation costs, both on average and for selected subgroups, for severe infection with the respiratory syncytial virus (RSV) in young children. METHODS: We collected data of children hospitalised for severe RSV infection during four seasons 1996/97, 1997/98, 1998/99, and 1999/2000 in 29 hospitals in the Southwest of The Netherlands (n=3487). Five patient characteristics (sex, gestational age, presence of bronchopulmonary disease (BPD), birth weight and age) were collected for all patients together with the number of hospital days. In addition, detailed information of 20 resources related to daily hospital care was collected for children hospitalised in the last season (1999/2000). Volumes of care were obtained from hospital registries and questionnaires among nurses and physicians. Prices were based on hospital accounts and national salary guidelines. For the last season (n=946), the individual total hospitalisation costs were directly calculated. We estimated the costs for the children hospitalised in the first three seasons (1996/97, 1997/98, and 1998/99) based on the correlation of costs with the number of hospital days and the 5 patient characteristics using a linear regression model. RESULTS: The number of hospital days accounted for >80% of the variety in total hospitalisation costs in the season 1999/2000. The average total costs of the season 1999/2000 were €3135 per child. The costs for children with low gestational age were considerably higher (€5325 for a child with gestational age