Measles has become epidemic over most of the world, with an important increase in the number of cases and associated morbidity and mortality in the United States since 1986. The two major factors responsible for this rise in the number of cases are, first, the increase in unvaccinated preschool-age children and, second, vaccine nonresponders (approximately 5%). The highest attack rate occurred in teenagers (15 to 19 years old) and in nine states (82% of cases). This situation has prompted revised immunization recommendations for those counties reporting more than five cases of measles among preschool-age children during each of the previous 5 years. In these counties, a first dose with monovalent measles vaccine is recommended at 9 months of age, followed by a second dose with measles, mumps, and rubella vaccine at 15 months of age, and revaccination of all children at the time of school entry. Recent publications regarding the use of vitamin A and certain antiviral agents are encouraging and are discussed in the manuscript. All cases of measles should be reported and investigated promptly. A good outbreak-control program will depend on the rapid recognition of the disease, a team approach, and prompt vaccination or IgG administration to susceptible persons.