Omega-3 fatty acid supplements have been widely marketed as a "treatment" for autism, but have not undergone controlled studies to rigorously evaluate their usefulness in treating the symptoms of autism. This study will carry out a double blind placebo controlled trial to determine the efficacy of omega-3 fatty acid supplementation in treating symptoms of autism. Participants in the trial will be children diagnosed with autism, randomly assigned to treatment and placebo groups. The treatment will involve 12 weeks of daily omega-3 fatty acid supplementation. Behavioral outcomes will be assessed by parents, teachers, and blinded clinicians to examine changes in mood and behavior following supplementation. This research will help support or disprove the usefulness of omega-3 fatty acids in the treatment of autism and allow better understanding of the mechanisms by which fatty acids may exert their effects.