The pathologic manifestations of Marfan's syndrome may appear in early infancy as indicated by the hemodynamic and angiocardiographic findings in 5-month-old twins. Mitral insufficiency represented the major cardiovascular malformation. This was shown to be due to elongated chordae tendineae and elongation with redundance of the anterior leaflet of the mitral valves in both of the twins at autopsy. Dilatation of the aortic sinuses of Valsalva and pulmonary trunk were also seen. Reports of the occurrence of the syndrome, a Mendelian dominant trait, in identical twins have been surprisingly infrequent.