The clinical course of nine patients, aged 14 to 49, who either at surgery or autopsy demonstrated myxomatous valvular transformation, has been described. Five females presented with symptomatic mitral insufficiency. Two of them had had endocarditis whereas in the others a heart murmur was detected during childhood. Three of the four men had a recent onset of aortic insufficiency associated in one case with aneurysm of the ascending aorta. The fourth experienced sequential arterial rupture and finally aortic dissection, his asymptomatic valvular lesion becoming manifest only at autopsy.The electrocardiographic, roentgenographic, angiographic, and catheterization findings resembled those seen in rheumatic heart disease, except that neither fibrosis, calcification, nor stenosis was found. The histologic appearance of the valve leaflets was identical to that which has been described, primarily as an incidental lesion, in the Marfan syndrome. Only one of the nine patients, however, demonstrated arachnodactyly. Skeletal mensuration in the remainder revealed a scattering of the many stigmata known to be associated with the Marfan syndrome. Nevertheless, arachnodactyly was diagnosed in at least one member of each of the six families available for investigation, although we did not encounter dislocation of the lens or an example of a complete Marfan syndrome. Erdheim's necrosis of the aortic or arterial media was described in four of the eight cases in which vascular biopsies were obtained. Prosthetic replacement in either the aortic or mitral area was carried out in the eight patients with signs of valvular insufficiency. There was one immediate and one late death. Reoperation was necessary in another case for dehiscence from the mitral annulus and twice in one more patient for the same complication at the aortic area. None demonstrated annular dilatation but isolated aneurysmal enlargement of either the ascending aorta, the membranous septum, or the sinus of Valsalva was encountered.Insufficiency apparently resulted because of valve prolapse from either structural fatigue, ruptured chordae, loss of substance, interference with coaptation, or supervening endocarditis. Myxomatous change may involve not only the chordae and the valve leaflets but also the annulus, since four prosthetic dehiscences occurred either in the early or late postoperative period in three of the eight operated upon. Experience with the one patient who showed aortic insufficiency and aneurysm of the ascending aorta suggests that valvular transformation may be the cause of incompetence in some of these cases rather than extension of the dilatation to the aortic ring.These studies indicate that myxomatous valvular transformation must be considered in the differential diagnosis of valvular insufficiency not only in patients known to have the Marfan syndrome or its various formes frustes, but also as an isolated idiopathic lesion in individuals not distinguishable by external appearance from the normal population. At the present time myxomatous valvular transformation can be suspected to be the cause of isolated valvular insufficiency if evidence for a rheumatic etiology is lacking, musculoskeletal signs of the Marfan syndrome are present, or Erdheim's necrosis is found in the arterial biopsy. There is a characteristic appearance at operation (floppy valve) but the presence of the lesion can only be established by histologic examination of the leaflets.