BACKGROUND: Influenza causes approximately 36,000 deaths per year in the United States despite the presence of an effective vaccine. This assessment of the value of influenza vaccination to the U.S. population is part of an update to the 2001 ranking of clinical preventive services recommended by the U.S. Preventive Services Task Force. The forthcoming ranking will include the new recommendation of the Advisory Committee on Immunization Practices to extend influenza vaccination to adults aged 50 to 64 years. METHODS: This service is evaluated on the two most important dimensions: burden of disease prevented and cost effectiveness. Study methods, described in a companion article, are designed to ensure consistency across many services. RESULTS: Over the lifetime of a birth cohort of 4 million, it is estimated that about 275,000 quality-adjusted life years (QALYs) would be saved if influenza vaccination were offered annually to all people after age 50. Eighty percent of the QALYs saved (220,000) would be achieved by offering the vaccine only to persons aged 65 and older. In year 2000 dollars, the cost effectiveness of influenza vaccination is $980 per QALY saved in persons aged 65 and older, and $28,000 per QALY saved in persons aged 50 to 64. When the costs of patient time and travel are excluded, the cost effectiveness ratio of vaccinating 50- to 64-year-olds decreases to $7200 per QALY saved, and vaccinating those aged 65 and older saves $17 per person vaccinated. CONCLUSIONS: Influenza vaccination is a high-impact, cost-effective service for persons aged 65 and older. Vaccinations are also cost effective for persons aged 50 to 64.