Objectives: To determine the rates of hospital admission for respiratory syncy tial virus (RSV) infection among children born at different gestational ages. To assess the theoretical impact of palivizumab prophylaxis on admissions for RSV infection. Design: Retrospective cohort study of children born in 1991-2000.Se tting: Tertiary care university hospital. Methods: Data on all children born dur ing the 10 year period were combined with information on laboratory confirmed RS V infections in these children until the end of 2002.The theoretical impact of palivizumab on RSV associated admissions was estimated by applying the current r ecommendations for prophylaxis to the study population and using the observed ra tes of admission in the calculations. Interventions: None. Main outcome measures : Rates of RSV infection and hospital admission in different subgroups of childr en. Results: Children with chronic lung disease (CLD) were admitted for RSV infe ction at a rate of 12.0%. The corresponding rates in children born at ≤28 or 29-32 weeks gest ation were 7.1 %and 6.8%respectively. Children born at ≤32 weeks gestation accounted for 6.6%of all admissions due to RSV. Of 580 children who would have met the criteria for palivizumab prophylaxis, 27 (4.6%) were admitted with RS V during the presumed prophylactic period. The number needed to treat to prevent one admission for RSV infectionwas 15 for childrenwith CLD (with a total cost o f e75 000) and 43 for children without CLD born at ≤32 weeks gestation (with a total cost of e215 000). Conclusions: The rates of hospital admission for RSV in fection in premature infants were substantially lower than those in most previou s reports from other countries. Determination of the local rates of RSV admissio ns in different groups of children would be useful in making decisions about the use of palivizumab. To assess the theoretical impact of palivizumab prophylaxis on admissions for RSV infection. Design: Retrospective cohort study of children born in 1991 2000: Sertiary care university hospital. Methods: Data on all children born dur ing the 10 year period were combined with information on laboratory confirmed RS V infections in these children until the end of 2002. The theoretical impact of palivizumab on RSV associated admissions was estimated by applying the current r ecommendations for prophylaxis to the study population and using the observed ra tes of admission in the calculations. Interventions: None. Main outcome measures: Rates of RSV infection and hospital admission in different subgroups of childr en. Results: Children with chronic lung disease (CLD) were admitted for RSV infe ction at a rate of 12.0%. The corres ponding rates in children born at ≤28 or 29-32 weeks gestation were 7.1% and 6.8% respectively. Children born at ≤32 weeks gestation accounted for 6.6% of all admissions due to RSV. Of 580 children who would have met the criteria The number needed to treat to prevent one admission for RSV infection was 15 for children with CLD (with a total cost of e75 000) and 43 for children without (for example, for palivizumab prophylaxis, 27 (4.6%) were admitted with RS V during the presumed prophylactic period. CLD born at ≤32 weeks gestation (with a total cost of e215 000). Conclusions: The rates of hospital admission for RSV in fection in premature infants were substantially lower than those in most previou reports from other countries. Determination of the local rates of RSV admissio ns in different groups of children would be useful in making decisions about the use of palivizumab.