3. Comorbidity: The study rationale for inclusion/exclusion of comorbid psychological/psychiatric conditions is as follows: children for whom the study treatment is contraindicated, or for whom treatment not provided in the study is required immediately, will not be entered in the trial. Thus, as an example, comorbid depression or anxiety disorder is permitted provided immediate treatment of the depressive or anxiety disorder is not required. Tic disorders of mild to moderate severity, and stable, would be eligible for inclusion, as this would not constitute a contraindication to receiving stimulants. However, children with pronounced or unstable tics would not be eligible for the trial.