To the Editor.— We are writing to notify health care providers that the risk of allergic reactions to the measles, mumps, rubella (MMR) vaccine may be more common that recognized previously.1,2 Epidemiologic data suggest that life-threatening allergic reactions to the single antigen measles or MMR vaccine are extremely rare, less than 71.6 reactions per one million doses.2 Studies performed on college campuses reported no significant difference in reactions between the vaccine recipients and the unvaccinated controls, and no allergic reactions were reported after measles vaccination.3,4