The assessment of attention deficit hyperactivity disorder (ADHD) in adults has been a source of controversy. The authors tested competing ideas by evaluating familial transmission among adult and nonadult relatives of ADHD children. They analyzed ADHD symptom data collected by structured interviews from the members of 280 ADHD and 242 non-ADHD families. For both past and current symptoms, both the boys' and girls' families showed significantly more familial aggregation for adult relatives than for nonadult relatives. The results were similar for inattentive and hyperactive-impulsive symptoms and for relatives with and without psychiatric comorbidity. The results provide further evidence for the validity of adult ADHD and support the intriguing idea that, from a familial perspective, the assessment of ADHD may be more valid in adults than in children. They do not support the idea that parents of ADHD children are biased to report ADHD symptoms in themselves because of their exposure to an ADHD child.