For children with autism spectrum disorder (ASD) and functional constipation (FC), their major gastrointestinal disturbance, there is a major knowledge gap in determining: 1) how different children respond to FC treatment; 2) specific ASD symptoms that are most responsive and stably improved when FC is treated; and 3) oxidative stress implications of ASD-FC co-morbidities and the ability to improve disrupted biological status. Children’s Hospital Los Angeles and the Center for Autism and Neurodevelopmental Disorders in Orange County will work together on the largest prospective study to date, evaluating 120 children with ASD with FC. Baseline medical and behavioral evaluations are performed, with follow-up visits at 3, 6 and 12 months for monitoring FC treatment effectiveness, measuring changes in ASD-related symptoms and determining pathophysiological status using the gold-standard biomarker for oxidative stress, F2 alpha Isoprostane, which is increased dramatically in children with ASD and FC and is a highly sensitive measure of improved pathophysiology state. This project will bring much needed insight for determining response heterogeneity to FC treatment, relation to pathophysiological disturbances, and the development of scalable strategies for predicting, at diagnosis, ASD symptoms that will be most amendable to improvement for each child