The patient was a 48-year-old woman who had been taking oral steroids for dermatomyositis since age 39. The patient experienced an episode of sudden chest tightness at age 48, and acute myocardial infarction was suspected. Coronary angiography revealed a right coronary artery occlusion ; emboli (thrombi) were collected from the same site. Left cardiac ventriculography revealed the presence of a mobile thrombotic mass in the ascending aorta. Although heparin therapy was initiated considering the possibility of thrombosis, no shrinkage of the mass was observed, and surgery was planned as per treatment guidelines. The mass was a rod-shaped thrombus measuring 20 mm×7 mm×7 mm attached to the aortic wall, approximately 2.5 cm distal from the entrance to the right coronary artery. Since the aortic wall at the site of the tumor attachment was normal, surgery involved only removal of the mass. A histopathological assessment revealed that the mass was a mixed thrombus containing both white and red thrombotic components. The patient was started on postoperative oral antiplatelet and anticoagulant drug therapy to prevent additional thrombosis, and no recurrence has been noted at 1 year postoperative.