A 63-year-old woman with progressive dyspnea underwent transthoracic echocardiography and was found to have a large multilobed mass in the left atrium that was attached to lateral wall. On inspection during surgery, the tumor was found to infiltrate the posterior mitral annulus and leaflet. The patient underwent surgical resection of the tumor and mitral valve replacement. Histologic and cytochemical evaluation confirmed that the tumor was a myxofibrosarcoma. Despite chemotherapy, the tumor recurred and the patient died 3 months after surgery.