In the United States, annual influenza epidemics typically occur during the late fall through early spring. During these epidemics, rates of serious illness and death are highest among adults aged ≥65 years, children aged <2 years, and persons of any age who have medical conditions that increase their risk for complications from influenza. Adults aged 50-64 years who have underlying medical conditions have a substantially increased risk for hospitalization during the influenza season. Influenza illness among healthy adults aged 18-64 years typically is not as severe as the illness among adults aged ≥65 years, pregnant women, or persons with chronic medical conditions and less frequently results in hospitalization. However, influenza among healthy adults aged 18-49 years is an important cause of outpatient medical visits and worker absenteeism. An economic analysis estimated an annual average of approximately 5 million illnesses, 2.4 million outpatient visits, 32,000 hospitalizations, and 680 deaths from influenza among adults aged 18-49 years who did not have a medical condition that increased their risk for influenza complications. In this analysis, adults aged 18-49 years accounted for 10% of the total economic cost from influenza, or approximately $8.7 billion.