This study investigated prevalence of life-threatening cardiovascular complications in the first 20 years of life of 186 patients with Marfan's syndrome. This was combined with echocardiographic observations on aortic root diameter. The aortic root diameter was measured in 91 of 186 patients (group A) and compared with that of 150 normal subjects aged 3 weeks to 20 years (group B). Fifteen patients underwent serial measurement of the aortic root diameter. Eight patients had serious cardiovascular sequelae that were related to the aorta (seven) and mitral regurgitation (one). In group A, aortic root diameters were at the upper limits of normal and above. Dilatation of the aortic root occurred in the eight patients before the development of life-threatening complications. Prevalence of serious cardiovascular manifestations of Marfan's syndrome in young persons is low. In children the aortic root diameter should be measured regularly (every 6 to 12 months) in as much as its rapid dilatation is predictive of those at risk of life-threatening sequelae.