Three patients with peripheral embolization and negative echocardiographic examinations were studied with MR imaging. Magnetic resonance imaging detected aortic arch thrombi in the absence of aortic aneurysm, dissection, or ulcerated plaques. By combining spin echo and cine MR sequences, MR enabled us to characterize and contrast intravascular mural thrombi reliably without application of contrast media. Thus we are assisted in the identification of patients at risk for embolization from an often overlooked source: the thoracic aorta.