Although the assertion of a link between vaccines and autism has been scientifically rejected, the theory continues to be popular and may influence the attitudes of parents of children with autism spectrum disorders. The authors sought to assess how often parents change or discontinue their child's vaccine schedule after autism spectrum disorder diagnosis and whether beliefs about the etiology of autism affect their decision to do so. The authors surveyed 197 (43%) of 460 eligible parents of children under 18 years of age with autism spectrum disorders who were enrolled in a state-funded agency that provides services to those with developmental disabilities in western Los Angeles County. Half of the parents discontinued or changed vaccination practices, and this was associated with a belief that vaccines contributed to autism spectrum disorders, indicating a potential subset of undervaccinated children. Educational tools should be designed to assist physicians when talking to parents of children with autism spectrum disorders about vaccination.