Primary pericardial sarcomas are extremely rare. The authors report a case of a 46-yr-old woman in whom a large mediastinal mass was discovered. The patient presented with cough, dyspnea, and orthopnea. Diagnostic investigations, such as echocardiography, computed tomography, and exploratory thoracotomy provided the evidence of a large mass in the mediastinum, attached by a broad base to the superior portion of the pericardium. A excisional biopsy was performed, and histologic examination of a biopsy specimen showed undifferentiated sarcoma. However, the complete removal of the mass was impossible due to adhesion to the adjacent great vessels. After the completion of the chemotherapy the patient was completely asymptomatic. However, follow-up transesophageal echocardiography showed a residual 3x4 cm-sized mass. The patient received the radiotherapy with a total dose of 55 Gy over 6 weeks. At present, there is no evidence of disease progression.