Errors in folate metabolism may play a role in the pathology of autism spectrum disorders because of increased vulnerability to oxidative stress. We report a case where L-methylfolate supplementation improved symptoms of aggression and disruptive behavior in a child with autism who tested positive for the C677TT allele of the methyltetrahydrofolate reductase enzyme gene. To our knowledge, this is the first report of L-methylfolate administration in this situation. Further controlled studies of L-methylfolate in this population are warranted. Copyright (C) 2017 Wolters Kluwer Health, Inc. All rights reserved.