Objective: To study the influence of increased familial risk for attention-deficit/hyperactivity disorder (ADHD) on brain morphology. Method: Volumetric cerebral measures based on whole brain magnetic resonance imaging scans from 30 boys with ADHD, 30 of their unaffected siblings, and 30 matched controls were compared. Results: Both subjects with ADHD and their unaffected siblings displayed reductions in right prefrontal gray matter and left occipital gray and white matter of up to 9.1% (p < .05). Right cerebellar volume was reduced by 4.9% in subjects with ADHD (p = .026) but not in their unaffected siblings (p = .308). A 4.0% reduction in intracranial volume was found in subjects with ADHD (p = .031), while a trend was observed in their unaffected siblings (p = .068). Conclusions: The volumetric reductions in cortical gray and white matter in subjects with ADHD are also present in their unaffected siblings, suggesting that they are related to an increased familial risk for the disorder. In contrast, the cerebellum is unaffected in siblings, suggesting that the reduction in volume observed in subjects with ADHD may be more directly related to the pathophysiology of this disorder.