New evidence suggests that autistic disorder (AD) 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 AD 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, we propose a randomized, placebo-controlled crossover pilot study with two phases. The first phase will focus on the behavioral and biochemical responses of children with AD to high-dose folic acid supplementation. Because FA is an inactive folate that requires biochemical conversion to become active, and select genotypes impede this conversion, our general hypothesis is that FA will yield behavioral improvements in some children but exacerbate problem behaviors 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. The focus here would similarly be on the behavioral and biochemical outcomes of participating children following treatment with the study supplement. Because Metafolin® is an active folate metabolite that should not be affected by genotypes in the folate pathway, our general hypothesis for phase 2 is that Metafolin® would 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 AD, potentially justifying the need to examine effects of folate supplementation among a larger sample of affected children.