The papillary muscles of the left ventricle, the related left ventricular wall, and the anterior papillary muscle of the right ventricle were studied for histologic evidence of and extensiveness of fibrosis. In addition to controls, in several conditions associated with left ventricular hypertrophy, necropsy specimens of heart without significant coronary disease from 84 cases including 45 from infants or children and 39 from adults with a variety of disease states were studied. The conditions included aortic stenosis, coarctation of the aorta, endocardial fibroelastosis, and left ventricular hypertrophy with failure. In none was coronary atherosclerosis present. In each condition studied, the degree of fibrosis observed in the left ventricular papillary muscles was greater than that observed in control hearts. There was direct relationship between severity of fibrosis and the angiographic demonstration of mitral insufficiency. The common denominator was left ventricular hypertrophy which appears responsible for the scarring of the left ventricular papillary muscles. Mitral valvular insufficiency may result in those cases with extensive scarring of the left ventricular papillary muscles.