Secondary endocardial fibrosis of variable severity was observed in the left ventricle in 13 of 16 patients after mitral valve replacement. Similar lesions were not observed in any of 20 patients who died in the late postoperative period after isolated aortic valve replacement. The left ventricular endocardial thickening is believed to be the result of turbulent flow, produced in the left ventricle by the caged ball within it. Severe impairment of cardiac function appeared to result from the process in two of the 13 patients: each developed intractable and ultimately fatal cardiac failure several months after double valve replacement, and the extensive endocardial thickening was the only anatomical cause for failure in both patients. Thus, secondary left ventricular endocardial fibrosis appears to be another, although uncommon, cause of cardiac failure in the late postoperative period after replacement of the mitral valve with a cagedball prosthesis.