Two cases of mitral insufficiency are reported. In 1 case the lesion was produced by rupture of the chordae tendineae of the posterior leaflet of the mitral valve. In the other case it was produced by fibrous adhesion of the posterior leaflet of the mitral valve to the underlying left ventricular wall, which occurred during healing of subacute bacterial endocarditis. Changes occurred in the pulmonary vascular bed which are essentially identical with those found in mitral stenosis.