The prevalence of autism in society makes research on its determinants and correlates particularly important. To rigorously quantify the strength of the connection between autism and three known comorbid conditions—epilepsy, allergies, and asthma—this article employs the child development survey, a large and nationally representative dataset. We find a persistent, statistically significant link between autism and epilepsy, and smaller correlations with allergies and asthma. This data provides a high level of statistical power that allows us to draw conclusions about a possible secular increase in autism. While the etiology of autism remains unclear, our results help clarify possible causative factors.