The largest measles outbreak in the United States during 1999 was traced to a 34-year-old minister with an undocumented history of vaccination, infected while traveling outside the United States. Local health departments in the Central Virginia Health District performed an epidemiological and laboratory investigation that identified 14 additional confirmed cases of measles, including 2 in health care providers and 5 in congregation members. Eight cases (53%) occurred among adults aged 30-35 years and 7 (47%) among children aged 13 months to 8 years. Although no religious exemptions were cited, only 2 case patients had documented proof of vaccination. This outbreak demonstrates the potential for limited indigenous spread of measles that occurs when imported cases expose susceptible groups. Almost half of the imported measles cases in the United States occur in US residents returning from foreign travel. Vaccination is highly recommended for all overseas travelers who are without documented proof of adequate immunization or measles immunity.