Participants in this double blind study will first receive open label stimulant treatment for ADHD and a comorbid disruptive behavior disorder. Participants whose aggressive behavior subsides with stimulant treatment alone will not proceed into the next phase of the study. Participants whose ADHD symptoms lessen from the treatment but whose aggressive behavior persists will be randomly assigned to receive either divalproex sodium or placebo in addition to their stimulant treatment for 8 weeks. Study visits will be held weekly for 11 to 16 weeks. At these visits, aggression levels and medication side effects will be assessed. Families will also meet with the researchers to discuss the child's progress, and attend behavioral counseling with a therapist. Participants who did not continue into the second phase of the study will be asked to return to the study site for a follow-up visit 8 weeks following the end of the first phase.