A 71-year-old man was admitted to our hospital with a 6-month history of progressive dyspnea on exertion. He had diabetes mellitus on medications. On physical examination, his pulse rate was 90 bpm and blood pressure was 100/70 mm Hg. A low-frequency diastolic heart murmur was heard at the apex without systolic murmur. Blood chemistry levels were within normal range. The 12-lead ECG showed a sinus rhythm with left atrium (LA) enlargement. Chest X-ray showed no abnormal finding. Transthoracic

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... ocardiography revealed a mobile, pedunculated mass (5.1ϫ3.2 cm) in the LA, which was attached to the interatrial septum ( Figure 1 and online-only Data Supplement Movies I, II, and III). The mass was composed of large cystic and small solid parts. This mass showed protrusion into the left ventricle during diastole resulting in obstruction of transmitral inflow and severe mitral stenosis. The LA was enlarged, and trivial mitral regurgitation was found. Severe resting pulmonary hyperten-sion and right ventricular dysfunction were observed. On transesophageal echocardiography, the size of the mass was 5.3ϫ3.2 cm, including the distal cystic portion (3.2ϫ2.8 cm; Figure 2 and online-only Data Supplement Movie IV). He underwent open heart surgery for mass removal, and the pedunculated mass was successfully excised from the LA. The mitral valve showed no morphological abnormality. Histopathological examination of the mass revealed that the solid portion was myxoma and the cystic portion was filled with blood (Figures 3 and 4) . Postoperative course was uneventful, and the patient was discharged on the eighth day after surgery. Myxoma is the most common primary cardiac neoplasm, accounting for nearly 50% of benign heart tumors. Typical myxomas are solid and gelatinous in consistency, with smooth or villous surface. Most myxomas appear as round and solid masses without cystic architecture or cavitation. Although several cases of cystic myxomas that had serous Figure 1 . Transthoracic echocardiography. A, A mobile pedunculated mass is shown in the left atrium, which was attached to the interatrial septum. The mass was composed of large cystic and small solid parts. B, On contrast echocardiography using perfluorocarbonexposed sonicated dextrose albumin, cystic mass can be clearly delineated. C, Color Doppler image shows diastolic transmitral inflow acceleration. D, Continuous-wave Doppler tracing shows a high transmitral mean pressure gradient (28 mm Hg). From the Division of Cardiology (