Respiratory syncytial virus (RSV) is an important cause of acute lower respiratory tract disease among the elderly, but national estimates of the burden of this disease have not been made. To estimate the morbidity, mortality, and medical costs of RSV-associated pneumonia among US elderly, national hospital discharge data, vital statistics, etiologic studies of adult pneumonia hospitalizations, and Medicare cost data were reviewed. In the United States, 687,000 hospitalizations and 74,000 deaths caused by pneumonia occur annually among the elderly; approximately 2%-9% of these are caused by RSV. At a cost of $11,000 per RSV pneumonia hospitalization, the estimated annual cost of RSV pneumonia hospitalizations is $150-$680 million. Exacerbations of congestive heart failure and other chronic conditions may also contribute substantially to RSV disease burden among the elderly. The total RSV disease burden is probably great enough to justify development of an RSV vaccine for use in this group.