We have studied four children varying in age from 5 months to 26 months with typical features of Marfan syndrome. Aside from the routine clinical and laboratory procedures, phonocardiograms, cardiac catheterizations, angiocardiograms and retrograde aortograms were performed. All patients had demonstrable aneurysms of the aortic sinuses of Valsalva and dilatation of the ascending aorta and of the main pulmonary artery. One case had a partial anomalous venous return to the innominate vein. The phonocardiograms demonstrated accentuated pulmonic closures, mid or late systolic murmurs and in one case an apical pansystolic murmur. The anomalies found at necropsy in one infant at the age of 12 days and other congenital heart defects in three older patients are reported. The general aspects of the disease are discussed and a review of the associated cardiac lesions in previously reported cases is presented. Attention is called to the minimal clinical findings of serious underlying cardiac lesions at a very early age, which can only be demonstrated by angiocardiography and retrograde aortography. To our knowledge these are the youngest patients in whom these cardiovascular lesions were demonstrated during life.