Introduction.Respiratory syncytial virus is a major cause of bronchiolitis during the first year of life. Preterm infants and patients with bronchopulmonary dysplasia (BPD) have a higher risk of hospitalization. The objective of this study was to assess the risk of hospitalization and to evaluate the cost effectiveness of palivizumab use in our at risk population.Methods.The study design was based on the hypothetical use of palivizumab in a cohort of patients. The cost effectiveness analysis included costs of palivizumab, the drug prescription and hospitalization. The effectiveness measure was the hospitalization rate decrease according to the result of the IMpact study: a 55% reduction in the need for hospitalization attributable to RSV. The years 1998 and 1999 were analyzed. Evaluable patients had follow-up at the High Risk Clinic and fulfilled the American Academy of Pediatrics criteria for palivizumab prophylaxis.Results.Forty-two patients, 24 with BPD and 18 preterm infants without BPD, were evaluated. Ten patients required hospitalization because of respiratory syncytial virus; one of them died. Hospitalization cost was $184 777. The total palivizumab administration cost would have been $185 064. With a 55% decrease of hospitalization rate, the cost per hospitalization averted would have been $15 358, and the number needed to treat would have been 7.9. A sensitivity analysis showed that the cost per hospitalization averted could range between $2171 and $48 630 according to the hospitalization rate.Conclusions.Because new effective but expensive treatments like palivizumab are available, this cost effective analysis can be an important tool in decisions about resource distribution.