The development of effective prophylaxis against influenza virus infections presents a special challenge. Stimulation of a protective response apparently requires more than one exposure to the virus. The virus may then change its surface antigens to render ineffective previously acquired immunity. Ideally, artificial prophylaxis should stimulate better protection than does natural infection and should anticipate antigenic change. Children experience the highest morbidity, accompanied by considerable risk, during influenza epidemics. At the same time, children in school and day care are the most important disseminators of virus in the community. Consideration should be given to providing protection for all children against influenza when effective methods and strategies have been found. Such prophylaxis would decrease not only morbidity and serious disease in children but also the risk of exposure of high-risk adult patients.