Fifteen patients have shown neovascularity in the left heart during selective coronary arteriography, with associated fistulous communication to a cardiac chamber in ten. In nine the abnormalities were adjacent to and within ventricular mural thrombi, associated with ventricular aneurysms in seven and congestive cardiomyopathy in two. In six patients with mitral stenosis and atrial fibrillation the angiographic changes related to left atrial thrombi. Histologic study of two atrial thrombi showed revascularization patterns in keeping both with the angiographic findings and with an intermediate stage in the evolution of mural thrombus. Though myxomas can produce a similar appearance, clinical and additional angiographic features should permit differentiation in most instances between this infrequent tumor and the far more common mural thrombus.