We report a case who had confirmed tumor cells in the biopsy specimens by transvenous endomyocardial biopsy with intra-procedural consultation and fast smear cytology. A 57-year-old female was admitted to our hospital because of shortness of breath and left back pain. Transthoracic echocardiography (TTE) and contrast-enhanced computed tomography (CT) scans demonstrated a large mass in the right atrium and multiple liver tumors thought to be due to spread of the disease. Coronary angiography showed the right coronary artery was involved in the mass. In order to confirm the histological diagnosis, we attempted transvenous endomyocardial tumor biopsy under fluoroscopic guidance. However, we failed to obtain adequate tissue material. Due to several risks associated with a surgical procedure such as an open surgical biopsy, transvenous endomyocardial tumor biopsy was again attempted with the aid of transesophageal echocardiography (TEE). Intra-procedural consultation and fast smear cytology enabled us to finish the procedure. Hematoxylin-eosin stained sections demonstrated spindle-shaped cells. Immunohistochemical stains of these cells were positive for anti-factor VIII antigen, CD31, and CD34. These findings indicated a definite diagnosis of angiosarcoma. Since there was no surgical indication for this tumor, the patient underwent chemotherapy with docetaxel and radiotherapy. Three months later, CT scans showed a reduction in the size of the cardiac tumor.