Background: Parental concerns persist that immunization increases the risk of autism spectrum disorder, resulting in the potential for reduced uptake by parents of younger siblings of children with autism spectrum disorder ("younger sibs"). Objective: To compare immunization uptake by parents for their younger child relative to their older child with autism spectrum disorder ("proband") and controls. Design: Immunization status was obtained for 98 "younger sibs," 98 "probands," and 65 controls. Results: A significant group difference emerged for overall immunization status (Fisher's exact test = 62.70, "p" < 0.001). One or more immunizations in 59/98 younger sibs were delayed (47/98; 48%) or declined (12/98; 12.2%); immunizations were delayed in 16/98 probands (16.3%) and declined in only one. All controls were fully immunized, with only 6 (9.2%) delayed. Within the "younger sibs" group, 25/98 received an autism spectrum disorder diagnosis; 7 of whom (28%) were fully immunized. The rates of autism spectrum disorder diagnosis did not differ between immunized and nonimmunized younger sib groups, although small sample size limits interpretability of this result. Conclusion: Parents who already have one child with autism spectrum disorder may delay or decline immunization for their younger children, potentially placing them at increased risk of preventable infectious diseases.