Attention deficit hyperactivity (ADHD) is a highly heritable behavioral disorder characterized by symptoms of inattention, hyperactivity, and/or impulsivity. Detection of susceptibility genes underlying ADHD may benefit from refinement of the ADHD phenotype into components that reflect more specific gene to behavior pathways and through the reduction of etiological heterogeneity. Using affected sibling pair (ASP) families, we are examining familial clustering of ADHD symptoms, neurocognitive task performance, and the occurrence of comorbid conditions to attempt to refine the phenotype and/or identify clinical features that may be used to stratify ADHD subjects to reduce etiological heterogeneity.