Vaccinations required for school attendance have increased nearly threefold since the 1950s, now targeting 17 infectious diseases. However, the impact of the expanded schedule on children’s overall health remains uncertain. Preliminary studies comparing vaccinated and unvaccinated children have reported that the vaccinated are significantly more likely than the unvaccinated to be diagnosed with bacterial infections, allergies, and neurodevelopmental disorders (NDDs). The objective of this study was to determine the association between vaccination and NDDs in 9- year-old children enrolled in the Medicaid program. The specific aims were to test the hypothesis that: 1) vaccination is associated with autism spectrum disorder (ASD) and other NDDs; 2) preterm birth coupled with vaccination increases the odds of NDDs compared to preterm birth without vaccination; and 3) increasing numbers of vaccinations are associated with increased risks of ASD.