Objective. To assess evidence for the use of RSV-IG with selected subgroups of high risk infants based on existing data from a randomized placebo-controlled trial. Methods. Number needed to treat (NNT) is a statistic of treatment effect, measuring the number of patients that would need to be treated to prevent one negative outcome. The NNT to prevent one negative outcome (RSV hospitalization) is computed for all 510 subjects in the PREVENT trial and for subgroups of subjects. Threshold number needed to treat(T-NNT) is the value representing equality between costs and benefits of preventing a negative outcome. T-NNT is computed according to a formula incorporating costs of hospitalization, willingness to pay to avoid a hospitalization, cost of RSV-IG, and willingness to pay to avoid adverse events attributable to RSV-IG administration. Results. More than 18(95% C.I. = 9.3, 500.0) infants would need to be treated to avoid one RSV hospitalization. Among subgroups of infants, this number ranges from 63 for premature infants without chronic lung disease to 7 (C.I. = 3.7, 41.7) for infants with bronchopulmonary dysplasia (BPD) who are 6 months of age or more. Based on sensitivity analysis of low, mid-range and high plausible assumptions of the threshold number needed to treat, there is strong inference that the costs and risks of RSV-IG treatment outweigh the benefits for the full sample of infants in the PREVENT trial and the following subgroups: premature infants without lung disease, infants with BPD who are 6 months of age or less, and all infants 6 months of age or less. Based on these data, treatment isnot recommended for these groups. Conclusions. T-NNT is sufficiently at odds with the data-based NNT under plausible assumptions to question recommendations for the use of RSV-IG with infants other than those with BPD who are older than 6 months of age. By stating assumptions about the costs and values of care explicitly, NNT procedures offer a methodology to grade data on the efficacy of RSV-IG for high-risk groups of infants. Simple decision rules can be used to accept or reject recommendations for treatment.