A 75-year-old male had a history of hypertension, diabetes mellitus, and smoking 20 cigarettes per day for 50 years. He had no history of atrial fibrillation. He underwent a medical checkup, and upper gastrointestinal endoscopy revealed middle thoracic esophageal squamous cell carcinoma. Since submucosal invasion by the cancer was suspected, surgery was planned after chemotherapy, and one course of FP therapy (5-fluorouracil, 800 mg/m2/day, continuously for 5 days, and cisplatin, 80 mg/m2, on day 1) was administered. Eleven days after the end of the first course of FP therapy, the patient underwent CECT to evaluate the effects and possible complications of the treatment. He was asymptomatic at that time, and his vital signs were unremarkable. Tests of the patient's coagulation status revealed a shortened activated partial thromboplastin time (21.2 s) and a high D-dimer level (1.6 mug/mL). CECT images obtained about 1 month earlier had not shown any abnormalities in the aorta (Figure 1); however, the new CT images incidentally revealed a floating mass in the ascending aorta (Figure 2). The mass exhibited uniform low density, but did not display enhancement. On the same day, 4D-CT was performed for further examination. The patient received 100 mL of 300 mg/mL iodinated contrast medium for the CECT and 65 mL of 370 mg/mL iodinated contrast medium for the 4D-CT; however, his renal function did not deteriorate. 4D-CT images showed that the mass was attached to the anterior wall of the ascending aorta along a region measuring about 18 mm in length. A 36-mm-long part of the mass was moving up and down, and the movement was synchronized with the patient's heartbeat (Figure 3(a), (b) and Video 1, 2). Since the appearance of the floating mass suggested that it was soft, we suspected a thrombus or benign tumor. Moreover, it seemed that the mass could become dislodged at any moment, which could have resulted in serious embolic complications. Therefore, the patient underwent emergency surgery the next day. Since malignancy could not be completely ruled out, replacement of the ascending aorta was performed. During the operation, the mobile mass was found to be attached to the anterior wall of the ascending aorta. A microscopic examination revealed that the mass was a fibrin thrombus, which was attached to the aortic wall. The aortic wall exhibited evidence of atherosclerosis; however, no continuity was found between the thrombus and tunica intima (Figure 4). The patient received heparin (5000 units) for 11 days postoperatively. His postoperative course was uneventful, and he was given an ambulatory discharge at 15 postoperative days. About 2 months after the discharge, he underwent surgery for esophageal cancer. He took 30 mg edoxaban as an oral anticoagulant for 188 days until his D-dimer level normalized, and the thrombus had not recurred after about a year.