The next step will be to enter these children into an open, two week trial to confirm their treatment resistance. Group 2, those treated with suboptimal doses, will have the dosage increased to the maximum recommended stimulant dose. Group 1, those who had been treated for 4 weeks with maximal doses of a stimulant, or Group 3, those who developed moderate to severe drug-related side effects, will be switched to the other class of stimulant for a two week trial, unless they have been tried on both classes. Those children will be maintained on their current class of stimulant for two weeks. At the end of the two weeks, all children will be rated on the ADHD-IV-RS by the Study Doctor. Those who have shown moderate to severe side effects or those who respond will exit the trial and be treated openly. Children from Group 2 who continue to show no improvement after a week will switch to the other stimulant. All children from Group 2 tried on both stimulants and all children from Groups 1 and 3 who continue to show mean scores greater than 1 SD over the mean for age will be referred to Phase 2.