Studies have shown that many children with autism exhibit signs of oxidative stress, which causes cellular damage by the presence of highly reactive free radicals. Antioxidants may protect against the damaging effects of oxidative stress, and low antioxidant levels have been observed in children with autism. Nutritional supplementation by injections of the antioxidant methyl B12 is a current alternative medicine treatment for children with autism which has anecdotal reports of clinical improvements. However, the efficacy of methyl B12 to treat symptoms of autism and reduce oxidative stress has not been tested in controlled clinical trials. This study will carry out an 8 week, double blind, placebo controlled clinical trial in 50 autistic children aged 3 to 8 years. Researchers will evaluate the subjects' behavioral responses to methyl B12 treatment. Additionally, they will monitor the blood levels of antioxidant metabolites, to determine whether methyl B12 treatment can reverse the signs of oxidative stress observed in autism. This study will help determine whether methyl B12 is an effective treatment for core features of autism, and may identify novel diagnostic markers for treatment responses in autistic subjects.