Marfan's syndrome (MS) and bicuspid aortic valves (BAVs) are associated with aortic dilation. Despite their histologic similarities, the 2 diseases differ with regard to the location of maximal aortic dilation. Echocardiographic analysis of aortic dimensions was performed in children with MS, children who had aortic dilation in the setting of nonstenotic BAVs (peak gradient <16 mm Hg), and healthy controls. Data on 25 patients with MS, 31 with BAVs, and 65 controls were analyzed. Subjects with BAVs had greater dimensions at all levels of the aortic root compared with normal controls. Patients with BAVs also had greater dilation than those with MS at the level of the annulus and the ascending aorta, with dilation often extending cranially beyond the region of measurement. Conversely, patients with MS had more focal dilation at the sinuses of Valsalva compared with controls and patients with BAVs. In conclusion, despite similar histologic abnormalities, the anatomic pattern of aortic dilation differs in children with MS and BAVs.