Attention deficit hyperactivity disorder (ADHD) is one of the most common mental disorders in children. Children with ADHD often have impaired functioning in home and school and usually experience difficulty relating to peers. If left untreated, the disorder can have long-term adverse effects into adolescence and adulthood. Atomoxetine is a selective noradrenergic reuptake inhibitor that is FDA-approved for the treatment of ADHD in children, adolescents, and adults. Unlike most other medications for ADHD, atomoxetine is not a stimulant. Studies have shown that atomoxetine is effective in treating ADHD, but more information is needed on its effectiveness in young children. This study will evaluate the effectiveness of atomoxetine in reducing the symptoms of ADHD in young children.