Small coronary artery fistulas terminating at the site of adherent, organized mural thrombi in the left atrial appendage were observed during selective coronary angiography in patients with mitral stenosis. The angiographic features of this abnormality can be distinguished from those of cardiac tumors, vascular malformations, and coronary artery fistulas that are not associated with organized thrombus. This coronary angiographic abnormality may indicate the presence of left atrial thrombus that is not revealed by echocardiography and is not manifest clinically by systemic emboli. The size of the collection of radiographic contrast material in the left atrium is not proportional to the size of the thrombus.