State and local health departments, in collaboration with CDC, continue to investigate a mumps outbreak that began in New York in June 2009. The index case occurred in a boy aged 11 years who had returned on June 17 from a trip to the United Kingdom, where approximately 7,400 reports of laboratory-confirmed mumps were received by the Health Protection Agency in 2009. He then attended a New York summer camp for tradition-observant Jewish boys, where he became symptomatic on June 28. Subsequently, other camp attendees and a staff member were reported to have mumps, and transmission continued in multiple locations when the camp attendees returned home. As of January 29, 2010, a total of 1,521 cases had been reported, with onset dates from June 28, 2009, through January 29, 2010, a substantial increase from the 179 cases reported as of October 30, 2009. The outbreak has remained confined primarily to the tradition-observant Jewish community, with <3% of cases occurring among persons outside the community. The largest percentage of cases (61%) has occurred among persons aged 7-18 years, and 76% of the patients are male. Among the patients for whom vaccination status was reported, 88% had received at least 1 dose of mumps-containing vaccine, and 75% had received 2 doses. This is the largest mumps outbreak that has occurred in the United States since 2006. Although mumps vaccination alone was not sufficient to prevent this outbreak, maintaining high measles, mumps, and rubella (MMR) vaccination coverage remains the most effective way to prevent outbreaks and limit their size when they occur.