Poor inhibitory control has been proposed to be central to the cognitive deficits and symptomatology associated with Attention Deficit Hyperactivity Disorder (ADHD). ADHD is a highly heritable disorder with an increased incidence among the siblings of affected individuals. In the current proposal we investigate the expression of genetic susceptibility for ADHD in brain functioning. We will study cognitive functioning in patients with ADHD, their unaffected siblings and healthy matched controls. Our aims are 1) to determine whether increased familial risk for ADHD is associated with differential patterns of brain activation compared to normally developing children, during the performance of tasks designed to probe cognitive functions that are compromised in ADHD and 2) to determine whether differential patterns of activation are similar for boys with ADHD and their unaffected siblings.