Observations were made on foreign white residents of Thailand presumably free of immunologic or other “background” factors that might influence the epidemiology, clinical features, or virology of dengue and chikungunya infection. Most patients were studied serologically; in selected instances we attempted to isolate virus. Despite high concurrent infection rates in Thais, not more than 1% of 627 Americans resident in Bangkok in 1962 and 1963 were infected with dengue or chikungunya viruses. The low rate of exposure to Aedes aegypti that this implies correlates with the observed absence of water storage in American houses. Primary dengue infections in American children were non-dengue-like; clinical misdiagnosis of dengue was higher in children than adults. A positive tourniquet test, low-grade thrombocytopenia, and hemorrhagic rashes were occasionally observed in patients with primary dengue infections. Among an estimated 500 infections in foreigners occurring during the period of study no case of dengue hemorrhagic fever was observed. In a “pure” outbreak of dengue type 1 among military personnel in Ubol, Thailand, nearly every man with serologic evidence of recent infection had overt disease. One hundred cases of dengue hemorrhagic fever occurred concurrently in indigenous children living in the same area. The observed low rate of dengue infection in foreigners in Thailand should result in an extremely low probability that a person will be infected with dengue viruses two times during an average residence period of 2 years. If the hypothesis that dengue hemorrhagic fever accompanies heterologous secondary dengue infections is assumed, the following epidemiologic pattern would be expected: dengue fever in foreigners—hemorrhagic fever in natives. Mild hemorrhagic manifestations observed during dengue illnesses in persons without antibody suggest that these are part of the “normal” response to dengue infection and not related to immunologic experience in the host.