 Autism is a complex neurodevelopmental disorder with early childhood onset characterized by impairments in communication, social interaction, and repetitive behavior. Due to the lack of known treatments for autism, many parents seek complementary and alternative medical CAM therapies hoping to help their affected child. Methylcobalamin methyl B12 is a commonly used CAM treatment that has anecdotal reports of remarkable clinical improvements with few side effects. Prior studies have found that children with autism have deficiencies in key metabolites and antioxidants which can be caused by methyl B12 deficiency; additional studies have shown that methyl B12 normalizes deficiencies in these metabolites and antioxidants. Based on these reports, a pilot study was conducted at UC Davis on the effect of methyl B12 on the behavioral and metabolic measures in children with autism. The preliminary results of 29 subjects revealed a subgroup of 9 responders to clinical behavior assessments. These responders also demonstrated significant improvement on the plasma measures of antioxidant capacity, suggesting methyl B12 improves symptoms in a subgroup of children with autism by increasing key antioxidants. The current study will have an 8 week double blind design with 50 subjects, designed to evaluate improvements from methyl B12 by using behavioral assessments and analysis of specific metabolites in the subjects blood. This study will determine whether methyl B12 will lead to benefits in any of the core features of autism, and will examine metabolic changes with the hope of potentially identifying a biomarker for treatment response in a subgroup of subjects. 