The mass prolapsed across the mitral valve, resulting in transmitral flow obstruction. Resection included partial thickness of the left atrial wall including endocardium and resection of a 8 × 4.5 × 4-cm mass; complete resection was not possible due to the proximity of the mass to the pulmonary veins. Frozen section revealed inadequate margins composed of spindle cell tumor consistent with sarcoma. It was positive for actin, lysozyme, vimentin, and α1-antichymotrypsin and negative for CD34, S-100 protein, and muscle-specific actin (HHF35); therefore, an intermediate- to high-grade tumor consistent with myofibroblastic sarcoma was confirmed. Postoperatively, a staging workup was completed that was found to be negative for metastasis. Due to the presence of positive margins, the patient received postoperative infusional chemotherapy. The chemotherapy regimen included adriamycin (20 mg/m2), ifosfamide (2,500 mg/m2), and mesna (approximately 2,500 mg) for four cycles, which was completed 5 months postoperatively. The patient remained disease free and was followed up with serial transesophageal echocardiographs.