A retrospective cohort study following more than 2.5 million American children found no association between the timing of a child's first MMR vaccine dose before age 2 and a later autism diagnosis. It asked whether receiving MMR earlier within the recommended window, rather than later, changed the likelihood of an autism diagnosis. Data from US electronic records show a post-2021 decline in timely MMR coverage and an increase in children with no MMR dose recorded by age 2. Whether receiving a first MMR dose earlier within the recommended window, between 11.5 and 24 months, was associated with a later autism diagnosis in 2.5 million US children. When is MMR recommended?