In the past decade influenza has become increasingly recognized as a serious disease and pandemic threat. Elderly persons, young children, and individuals with chronic medical conditions have the greatest risk for complications or death from influenza infection. Neuraminidase inhibitors are currently licensed for the treatment and prevention of influenza if started early in the course of illness, but little is known regarding the effects of oseltamivir (one neuraminidase inhibitor) on illness severity when initiated later in the course of illness. Greater knowledge of the treatment effects is urgently needed for optimal management of seasonal influenza, and to maximize use of a limited stockpile of antiviral drugs in the event of an influenza pandemic.