 Self-injurious behavior SIB in children with autism spectrum disorder ASD can cause physical harm to the child and interfere with the childs ability to make use of educational programs and helpful treatments such as speech therapy. The turmoil caused by self-injurious behaviors in children with ASD invariably interferes with daily routines because family life often stops during these episodes and family members worry about setting off SIB between episodes. This project will use the detailed assessment methods developed in the field of behavior therapy to evaluate the potential for N-acetylcysteine NAC to treat children with ASD and moderate repetitive SIB. NAC is an over-the-counter dietary supplement that may have beneficial effects on the brain through its well-documented antioxidant effects andor reduced glutamate signaling. In the proposed study, 14 children with ASD and repetitive SIB between the ages of 5 and 12 will be randomly assigned to gradually increasing doses of NAC or placebo for 9 weeks. The research team, parents and children will be blind to the treatment with NAC or placebo. Participants will come to the research site periodically to complete measures and behavioral assessments. 