• An outbreak of measles presented an opportunity to examine the immune response of children initially immunized with measles vaccine before 1 year of age to reimmunization at 15 months of age with measles, mumps, and rubella vaccine. Eighteen previously immunized children had higher acute and convalescent hemagglutination inhibition titers than 13 control children. None of the previously immunized children had measles-specific IgM appear in convalescent serum samples. These results indicate a brisk secondary immune response to measles reimmunization in children immunized before 1 year of age. They support the recommendation of the American Academy of Pediatrics of reimmunization at 15 months of age for those children whose initial measles immunization was before 1 year of age.