An ever-increasing population of immunocompromised individuals, including not only the burgeoning AIDS population but also those patients whose host defenses have been damaged by a wide variety of diseases and their therapies, requires prophylaxis against infectious diseases. Existing vaccines can provide some benefit, but the clinician must always be alert to the fact that vaccine response cannot be assumed and such adjunctive measures as antimicrobial prophylaxis (i.e., amantadine during community-wide influenza outbreaks or penicillin prophylaxis against pneumococcal infection in certain asplenics) and/or intravenous immunoglobulin administration should be considered in some patients. In the future, in addition to developing new vaccines, a major effort should be devoted to the development of immunoadjuvants that would increase the effectiveness of vaccine administration.