Dengue fever is an acute, mosquito-transmitted viral disease characterized by fever, headache, arthralgia, myalgia, rash, nausea, and vomiting. Infections are caused by any of four virus serotypes (DEN-1, DEN-2, DEN-3, and DEN-4). The incidence of dengue is increasing in most tropical areas throughout the world (Fig. 1). Although dengue is not endemic in the continental United States, Hawaii, or Alaska, more than 500 laboratory-positive cases of introduced dengue were reported from 1977 through 1994 in U.S. residents who visited dengue-endemic areas throughout the world.1-4 In addition, two competent mosquito vectors (Aedes aegypti and Aedes albopictus) are found in the southeastern United States, and both could possibly transmit an introduced virus. In Hawaii, Ae. albopictus is the dominant mosquito on all islands; Ae. aegypti has only focal distribution on Molokai and the Kona coast of Hawaii. Economic, political, technologic, ecologic, and demographic changes have brought about the emergence of new microbial diseases, as well as an increase in the incidence of previously known infections. The increase in dengue activity in Asia, Africa, and the Americas represents a pandemic that is being facilitated by increased air travel; global urbanization; population growth; greater abundance of disposable, nondegradable containers that can serve as Aedes production sites; and lack of effective mosquito control programs.5,6 This report summarizes information about risk factors for severe disease, recent dengue outbreaks throughout the world, and cases of dengue virus infection in travelers who have been diagnosed on return to the United States.