Measles, a highly infectious viral illness, is no longer endemic in the United States because of high coverage rates with an effective vaccine. However, imported cases continue to cause illness and outbreaks among susceptible U.S. residents. In August 2007, a participant in an international youth sporting event who traveled from Japan to the United States became ill with measles. Because he traveled while infectious to an event with thousands of participants and spectators, an outbreak investigation was conducted in multiple states by state and local health departments in coordination with CDC, using standard measles surveillance case definitions and classifications. This report summarizes the results of that investigation, which identified six additional measles cases that were linked epidemiologically to the index case and two generations of secondary transmission. Viral genotyping supported a single chain of transmission; six of the seven cases were linked by genetic sequencing. U.S. organizers of large-scale events attended by international travelers, especially youths, should consider documentation of adequate participant vaccination. This outbreak highlights the need to maintain the highest possible vaccination coverage in the United States, along with disease surveillance and outbreak-containment capabilities.