New evidence suggests that autism spectrum disorder (ASD) may be associated with abnormalities in folate metabolism, which is a process that affects genetic expression by facilitating the formation of methyl donors for DNA methylation. Limited data show that some children with ASD show behavioral improvements with folic acid (FA) therapy, while others show a worsening effect. If behavioral worsening is linked with abnormalities in folate metabolism, then nutritional modifications could normalize these processes and result in clinical improvements. To address this premise, a randomized, placebo-controlled crossover pilot study will be conducted. The first phase of the study will focus on the behavioral and biochemical responses of children with ASD to high-dose folic acid supplementation. Because FA requires biochemical conversion to become active, and select genotypes impede this conversion, behavioral improvements may be observed in some children while increased problem behaviors may occur in others. During the second phase, children who had a worsened behavioral response to FA during phase 1 will participate in an open-label trial of high-dose Metafolin® supplementation. Because Metafolin® is an active folate metabolite that should not be affected by genotypes in the folate pathway, this supplement may yield behavioral improvements without the risk for behavioral worsening. Results from this project may provide support for continued study of the potential relationship between folate metabolism and problem behaviors among children with ASD.