This report describes the clinical course and serial echocardiographic abnormalities in a patient with nonbacterial thrombotic endocarditis and repeated embolic episodes. Serial echocardiograms revealed a mass of abnormal echoes on the anterior mitral leaflet and progressive restriction of mitral valvular motion. Cineangiographic studies demonstrated a large filling defect in the area of the mitral valve. Surgical intervention confirmed the presence of a large vegetation on the mitral valve. The surgical specimen consisted of a sterile, partially organized fibrin thrombus. The echocardiographic abnormalities are described and discussed in relation to the clinical and pathologic findings.