Chikungunya virus (CHIKV) is an alphavirus indigenous to tropical Africa and Asia, where it is transmitted to humans by the bite of infected mosquitoes, usually of the genus Aedes. Chikungunya (CHIK) fever, the disease caused by CHIKV, was first recognized in epidemic form in East Africa during 1952-1953. The word "chikungunya" is thought to derive from description in local dialect of the contorted posture of patients afflicted with the severe joint pain associated with this disease. Because CHIK fever epidemics are sustained by human-mosquito-human transmission, the epidemic cycle is similar to those of dengue and urban yellow fever. Large outbreaks of CHIK fever have been reported recently on several islands in the Indian Ocean and in India. In 2006, CHIK fever cases also have been reported in travelers returning from known outbreak areas to Europe, Canada, the Caribbean (Martinique), and South America (French Guyana). During 2005-2006, 12 cases of CHIK fever were diagnosed serologically and virologically at CDC in travelers who arrived in the United States from areas known to be epidemic or endemic for CHIK fever. This report describes four of these cases and provides guidance to health-care providers. Clinicians should be alert for additional cases among travelers, and public health officials should be alert to evidence of local transmission of chikungunya virus (CHIKV), introduced through infection of local mosquitoes by a person with viremia.