Using two sources of data, we review methodologic issues pertinent to family studies of attention deficit hyperactivity disorder to evaluate whether such studies define attention deficit hyperactivity disorder as a familial disorder. We systematically evaluate the relevant literature and provide a detailed overview of the Massachusetts General Hospital family-genetic studies of attention deficit disorder as defined in DSM-III and attention deficit hyperactivity disorder as defined in DSM-III-R. The available literature, and our double-blind, controlled studies indicate that attention deficit disorder and attention deficit hyperactivity disorder are familial. Moreover, the pattern of transmission of comorbid disorders suggests that attention deficit hyperactivity disorder is, from a familial perspective, distinct from anxiety disorders and learning disabilities. In contrast, attention deficit hyperactivity disorder with conduct disorder appears to be a familial subtype, and major depression appears to be a variable expression of the familial predisposition to attention deficit hyperactivity disorder. Although the available literature provides strong evidence for the familial transmission of attention deficit hyperactivity disorder, the mode of transmission requires further clarification. In addition, attention deficit hyperactivity disorder appears to be genetically heterogeneous, indicating that more work is needed to delineate genetically homogeneous subtypes and to describe the range of expression of their underlying genotypes. Family-genetic studies will continue to clarify the etiology and nosology of attention deficit hyperactivity disorder.