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Infants younger than 1 year, and especially premature infants, are susceptible to severe RSV disease, which accounts for up to 126,000 hospitalizations each year. In the United States, the RSV season generally extends from November though March, but varies considerably both temporally and geographically. Studies, such as the one we have conducted in Colorado, indicate that the RSV season can even vary among local communities. In addition to seasonal variability, other independent risk factors also have to be taken into consideration when considering RSV immunoprophylaxis. Among these are a GA of less than 33 weeks, infants with CLD or CHD or who are immunosuppressed, male sex, household crowding, and daycare attendance. Exposure to indoor tobacco smoke and abbreviated breast feeding (less than 2 months) also may be pertinent risk factors. The Colorado studies show that high altitude also increases the risk of RSV hospitalization. Although hospitalization is a major outcome of severe RSV infections in infants, the development of wheezing and physician-diagnosed asthma later in life may be a long-term outcome that should be considered.