Medication phase: After screening assessments are completed, the child will enter the medication phase of the study. The child will be started on atomoxetine at 0.5 mg/kg/day, with the dosage increased to a maximum dose of 1.8 mg/kg/day. The dose will be determined by how well the child responds to and tolerates the drug. The dose will be given twice a day to minimize side effects. After the optimum dose is determined, the child will be kept at this stable dose for 4 weeks.