To confirm or deny the thesis that mitral anular dilatation is a cause of mitral regurgitation, this anulus was measured in 102 patients, 24 of whom had normal hearts and 78 of whom had dilated left ventricles, some associated with mitral regurgitation, others not. The circumference of the mitral anulus in the 24 patients with normal hearts averaged 9 cm (range 7 to 11 cm); in 24 patients with idiopathic cardiomyopathy of the ventricular dilated type, both with (12 patients) and without (12 patients) mild to moderate regurgitation, 11 cm (range 10 to 14 cm); and in 31 patients with severe mitral regurgitation, 12 cm (range 8 to 18 cm). Among the latter 31 patients with severe mitral regurgitation, the anulus was only mildly dilated in the 15 with rheumatic disease (average 11 cm) and in the six with ruptured chordae tendinease on previously normal valves (average 10 cm); in contrast, in the 10 patients with floppy mitral valves with or without the Marfan syndrome the mitral anulus was greatly dilated (average 15.5 cm). Thus, only in patients with floppy valves with or without the Marfan syndrome or both does anular dilatation by itself appear great enough to cause mitral regurgitation. It appears, therefore, that dilatation of the mitral anulus is a rare cause of mitral regurgitation and that abnormality of the fibrous skeleton of the heart is necessary for the mitral anulus to dilate enough to cause mitral regurgitation.