The aim of the present study was to evaluate the efficacy of divalproex sodium treatment in adults with intellectual disability, and aggressive or self-injurious behaviour. Twenty-eight adults aged between 20 and 63 years of age with severe, long-lasting behavioural problems were treated with divalproex sodium (dosage 500-4000 mg day−1). Clinical changes were assessed at 2-73 months into the pharmacological treatment utilizing the Clinical Global Impression Severity (CGI-S) scale, and monthly behavioural counts of aggressive and self-injurious acts. Seventy-one percent of subjects demonstrated a moderate or marked improvement on the CGI-S; another 21% demonstrated mild benefits. Among the patients for whom objective prospective behavioural counts were available, 88% showed a significant reduction in aggression and self-injurious behaviour, 46% had other psychotropic medications discontinued, and another 39% had psychotropic medications decreased. One patient had serious thrombocytopenia which required the discontinuation of divalproex sodium, and one other had vomiting and worsened aggression. The present preliminary, uncontrolled study suggests that adults with intellectual disability, and aggressive or self-injurious behaviour may respond to divalproex sodium, and that this drug is well-tolerated in the majority of subjects.