Malaria has had a major influence on military campaigns for thousands of years. In this paper we summarize the experience of U.S. Navy and Marine forces with malaria during wars of the twentieth century. During World War I, there were 4,746 new cases of malaria, 68,373 sick-days because of malaria, and 7 deaths due to malaria; during World War II, there were 113,256 new cases, 3,310,800 sick-days, and 90 deaths; and during the Korean War, there were 4,542 new cases, 50,924 sick-days, and no deaths--since most infections were with Plasmodium vivax. During the Vietnam War, there were 24,606 cases of malaria, an estimated 391,965 sick-days because of malaria, and 46 deaths due to malaria. With the worldwide resurgence of malaria, the spread of drug-resistant strains of Plasmodium falciparum, the emergence of chloroquine-resistant P. vivax, and the increasing resistance of Anopheles mosquitoes to insecticides, malaria continues to be an enormous threat to U.S. Navy and Marine Corps personnel deployed to the tropics and subtropics.