Primary cardiac sarcomas are extremely rare. We report a case of a primary cardiac sarcoma with myxoid change, which originally presented as a benign cardiac myxoma on a two-dimensional echocardiogram. On operating, the mass was found to extend into the posterior left atrial wall, the left pulmonary vein, and the mitral valve. The patient underwent wide resection of the left atrium, a mitral valve replacement and a left pneumonectomy. The histological diagnosis was of an undifferentiated primary cardiac sarcoma. The patient had postoperative chemotherapy. The patient expired 11 months after surgery due to a recurrence of the cardiac sarcoma. Although most tumors that develop in the left atrium are benign myxomas, we should make a preoperative differential diagnosis.