Twelve (39%) of the 31 mumps cases were among campers. All were U.S. residents aged 10-15 years who had been vaccinated with 2 doses of measles, mumps, and rubella (MMR) vaccine after the first birthday. Nineteen (61%) of the mumps cases were among staff members; of these, nine (47%) were UK residents, five (26%) were U.S. residents, three (16%) were residents of Australia, and two (11%) were residents of Germany. Staff members with mumps ranged in age from 19 to 41 years (median: 21 years). Of the 17 staff members with mumps for whom vaccination history could be obtained by vaccination or medical record, nine (53%) had not been vaccinated for mumps, four (24%) had been vaccinated with 1 dose, and four (24%) had been vaccinated with 2 doses of a mumps-containing vaccine. Symptoms, illness duration, and complications (e.g., orchitis) did not differ substantially between vaccinated and unvaccinated patients.