Measles has not been endemic in the United States since 1997, although limited outbreaks continue to be caused by imported cases. In 2003, CDC assisted in investigating the largest school outbreak of measles in the United States since 1998. The outbreak consisted of 11 laboratory-confirmed cases: nine cases in a boarding school in eastern Pennsylvania and two epidemiologically linked cases in New York City (NYC). This report summarizes the results of the outbreak investigation, which indicated that measles continues to be imported into the United States and that high coverage with 2 doses of measles-containing vaccine (MCV) among students was effective in limiting the size of the outbreak. Health-care providers should maintain a high index of suspicion for measles, especially in those who have traveled abroad recently, and recommendations for 2 doses of MCV in all school-aged children should be followed.