We report two cases of imported infection in patients who had returned to Taiwan from Singapore: one was coinfected with chikungunya virus and dengue virus type 2, and the other was infected with the same dengue virus. Both viruses were successfully isolated from the coinfected case by using antibody neutralization and a plaque purification technique.Dengue fever, caused by a flavivirus in the family Flaviviridae, is the most prevalent arboviral disease in tropical and subtropical regions of Asia, the Pacific and Caribbean islands, and Central and South America (9). Chikungunya, caused by an alphavirus in the family Togaviridae, is endemic to Africa and Asia (12). Both diseases are transmitted to humans by day-biting Aedes aegypti and Aedes albopictus mosquitoes, and both diseases have similar clinical symptoms, including fever, rash, and joint pains as well as headache, fatigue, nausea, vomiting, and muscle pain; a laboratory test is required to distinguish between the two diseases. Thus, many risk factors for chikungunya virus (CHIKV) and dengue virus (DENV) infections are the same or similar. The urban mosquito Aedes aegypti is the primary vector of both viruses throughout most of their geographic range, although Aedes albopictus was recently identified as the main vector of the recently emerged CHIKV E1-226V variant of the African genotype (17).The explosive epidemics of chikungunya in Indian Ocean islands and India since 2004 and the worldwide increase in travel have facilitated the expansion of different strains of CHIKV of the African genotype into overlap areas where DENV is endemic (13). As a result, cocirculation of CHIKV and DENV has been reported in various geographic areas, including India, Sri Lanka, Gabon, Cameroon, Madagascar, Malaysia, Indonesia, Singapore, and Thailand. Consequently, a few studies showing patients coinfected with CHIKV and DENV have been reported in India, Sri Lanka, Malaysia, and Gabon (1,5,8,11,14). Although molecular and serologic evidence demonstrated or suggested coinfections in the abovementioned reports, neither CHIKV nor DENV was isolated from these patients. Successful isolation of both viruses is needed to conduct basic and applied research on CHIKV and DENV biology, immunology, and pathogenesis, as well as the development of laboratory diagnosis, antiviral drugs, and vaccines.The first and only concurrent isolation of CHIKV and DENV-2, from a single blood specimen taken from a patient in the acute phase of a dengue-like illness in southern India in 1964, was reported by Myers and Carey (10). In their study, the dominance of CHIKV in the coinfected patient's serum, along with growth competition, prevented the initial isolation of DENV-2; isolation was finally accomplished through pretreatment of the acute-phase serum sample with a CHIKV-specific mouse antibody, followed by inoculation into infant mice for in vivo growth. Here we report only the second case confirmed by actual isolation of CHIKV and DENV-2, from a patient returned from Singapore, using an in vitro cell...