The clinical appearance, typical localization, diagnostic procedure and the treatment of hemangiosarcoma--the most frequent malignant tumor of the heart are reported by the case of a 27 years old woman. The patient suffered from dyspnea, congestion of the superior caval vein and paroxysmal tachycardia. X-ray showed cardiac enlargement due to pericardial effusion. Echocardiography revealed a large tumor in the right atrium. Computertomography and angiocardiography showed tumor masses at the orifice of the superior caval vein and a bypass of the blood flow via the azygos vein. Thoracotomy yielded an inoperable hemangiosarcoma. By the combined treatment of irradiation and chemotherapy the cardiac tumor completely disappeared, the patient was temporary symptomless. Later metastases occurred and the patient died 13 months after diagnosis.