A 68-year old man presented with a 1-month history of shortness of breath. He was a heavy smoker. His blood pressure was normal but his pulse rate was 40 beats per minute. A chest X-ray showed normal cardiac size without abnormal pulmonary lesions. The electrocardiogram showed sinus tachycardia with complete AV block (Fig. 1A). He was scheduled to undergo permanent pacemaker implantation because of complete AV block associated with dyspnea. However, a somewhat echolucent inhomogenous mass involving the interatrial septum and protruding toward both atria was found on an echocardiogram (Fig. 1B). Chest computed tomography revealed a large non-enhanced hypodense mass invading the atria, the ventricles, the pulmonary trunk, and the left main bronchus, suggesting a malignant tumor of cardiacorigin such as an angiosarcoma,1) fibrosarcoma,2) rhabdomyosarcoma, leiomyosarcoma,3)4) or metastatic cancer such as from a lymphoma (Fig. 1C).5) Histologic confirmation was not done because of the patient's refusal. Two month later, he died of progressively worsened heart failure caused by rapid tumor growth.