Attention Deficit Hyperactivity Disorder (ADHD) is a heritable psychiatric disorder with onset in childhood. Twin and adoption studies indicate that additive genetic factors explain up to 80% of the variance underlying susceptibility. The siblings of children with ADHD have a three- to fivefold increased risk of having ADHD compared to the siblings of healthy control subjects, and the risk is even greater for monozygotic twins with 50-80% concordance compared with up to 33% in dizygotic twins). As full siblings share on average 50% of their genes, even the unaffected siblings of children with ADHD would be expected to share some of the genes involved in the disorder. The neuroanatomical substrate of ADHD is becoming increasingly better defined by a growing body of evidence from imaging studies. Evidence from neuroimaging studies suggests that this disorder is associated with reductions in brain volume up to 5% in these children. In this protocol we collected MRI-scans from boys with ADHD and their unaffected siblings, as well as control subjects. In addition, cheekswabs were later collected for DNA analysis.