The clinical and echocardiographic manifestations in 25 patients with Marfan's syndrome diagnosed during infancy and childhood (mean [+/- SD] age, 8.1 +/- 4.8 years; range 0 to 16 years) were evaluated. Twenty-one patients (84%) had a midsystolic click, 11 patients (44%) had mitral regurgitation (MR), and five patients (20%) had combined MR and aortic regurgitation (AR). Echocardiography demonstrated mitral valve prolapse in all 25 patients, aortic root dilatation in 20 patients (80%), AR in seven patients (28%), and aortic aneurysm in five patients (20%). During the follow-up period (mean, 5 +/- 4.5 years), progressive AR and aortic aneurysm were documented in four patients, progressive MR in three patients, and progressive aortic root dilatation in two patients. Five patients (22%) died during the follow-up period. Among patients with a positive family history of Marfan's syndrome, MR was less frequent as compared with sporadic cases (29.4% vs 75%, respectively). Progressive cardiovascular involvement was more frequent among patients diagnosed before 10 years of age compared with those diagnosed later (60% vs 12.5%, respectively). Cardiovascular involvement was a common feature of childhood Marfan's syndrome, causing significant morbidity and mortality. Sporadic cases and children diagnosed before 10 years of age represented a particularly high-risk group.