ADHD is one of the most common childhood mental disorders. It often causes impaired functioning in multiple areas, including home, school, and peer relationships. Additionally, children with ADHD often develop aggressive behavior, which is not usually adequately suppressed by standard stimulant treatments for ADHD. In order to address this problem, many physicians prescribe multiple medications at once. There is no clinical evidence, however, proving that this method is safe and effective. This study will assess whether adding a mood stabilizer, divalproex sodium, to stimulant treatment is more effective than stimulant treatment alone in reducing aggressive behavior among children with ADHD.