Dengue is caused by four antigenically related viruses (DENV-1, DENV-2, DENV-3, and DENV-4). Dengue fever is endemic in most tropical and subtropical areas of the world, and in 2007 nearly 1 million cases were reported in the Americas alone. Dengue infections commonly occur among U.S. residents returning from travel to endemic areas and are more prevalent than malaria among returning travelers from the Caribbean, South America, South Central Asia, and Southeast Asia. This report summarizes information about dengue cases reported to CDC through two CDC-maintained passive surveillance systems: 1) the ArboNET surveillance system, a national CDC arboviral surveillance system maintained by CDC's Arboviral Diseases Branch and initially developed in response to the introduction of West Nile virus in the United States, and 2) a system maintained for decades by the CDC Dengue Branch (CDCDB), which collects information on all suspected dengue cases whose specimens are sent to the branch. During 2006-2008, a total of 1,125 unique reports were made to either ArboNET or CDCDB. Of these, the highest proportion of laboratory-confirmed and probable cases with known travel histories were in persons who reported travel to the Dominican Republic (121; 20%), Mexico (55; 9%), and India (43; 7%). Health-care providers should consider dengue in the differential diagnosis of patients with a history of travel to endemic areas within 14 days of fever onset.