 The purpose of this study is to demonstrate the feasibility of a 9-week, randomized trial of N-acetylcysteine NAC compared to placebo in 14 children age 5 to 12 years with Autism Spectrum Disorder ASD and a moderate level of repetitive self-injurious behavior SIB. Additional aims are to evaluate the positive predictive value of a screening method to classify children with automatically maintained self-injurious behavior; to evaluate the preliminary efficacy of NAC for reducing repetitive SIB in children with ASD; and to evaluate biomarkers and possible mechanisms of action of NAC in children with ASD. 