Autism is a neurodevelopmental disorder characterized by impairment in three core symptom domains: socialization, communication, and repetitive/stereotyped behaviours. Other associated symptom domains are also affected including impulsivity/aggression, self-injury, anxiety, and mood lability. Divalproex has been shown to have efficacy in treating epilepsy, bipolar disorder, mood lability, and impulsive aggression. The present study evaluated the use of divalproex in the treatment of repetitive, compulsive-like symptoms of autism spectrum disorder (ASD). Thirteen individuals with ASD participated in an 8-wk, double-blind, placebo-controlled trial of divalproex sodium vs. placebo. There was a significant group difference on improvement in repetitive behaviours as measured by the Children's Yale–Brown Obsessive Compulsive Scale (C-YBOCS) (p=0.037) and a large effect size (d=1.616). This study provides preliminary support for the use of divalproex in treating repetitive behaviours in ASD. Further research is needed to evaluate the specificity and mechanism of action of these findings.