The present report describes the case of a 61-year-old woman with malignant fibrous histiocytoma of the left atrium originating from the left atrial free wall, operated on in emergency for a suspected large left atrial myxoma that, at the echo scan, was consistently protruding through the left atrioventricular orifice at each diastole and was almost completely occluding the left ventricular inflow, causing signs of congestive heart failure and severe dyspnea. Surgery was performed as radically as possible, but the histological examination of the specimen revealed the exact diagnosis of the neoplasm. About 75% of primary tumors are benign and 75% of these are atrial myxomas. The malignant tumors consist of various sarcomas: myxosarcoma, liposarcoma, angiosarcoma, fibrosarcoma, leiomyosarcoma, osteosarcoma, synovial sarcoma, rhabdomyosarcoma, undifferentiated sarcoma, reticulum cell sarcoma, neurofibrosarcoma, and malignant fibrous histiocytoma. The long-term results for sarcomas are very poor and there are few survivors after several months from surgery due to the extent of local spread and invasion or because of the frequent distant metastases. Malignant fibrous histiocytoma constitutes about 2% of all cardiac malignancies, which might grow within several localized areas, occasionally in the heart. Echocardiography represents the best examination procedure for both diagnosis and follow-up of patients with cardiac tumors.