Dengue is a major cause of illness among travelers and a threat to military troops operating in areas to which it is endemic. Before and during World War II, dengue frequently occurred in US military personnel in Asia and the South Pacific. From the 1960s into the 1990s, dengue often occurred in US troops in Vietnam, the Philippines, Somalia, and Haiti. We found attack rates as high as 80% and periods of convalescence up to 3-1/2 weeks beyond the acute illness. The increase in dengue throughout the world suggests that it will remain a problem for military personnel until an effective vaccine is licensed.