Myxoma and other neoplasms in the left atrium often show signs and symptoms similar to those of mitral valve stenosis. Malignant fibrous histiocytoma, one of the most common sarcomas, occurs in the deep soft tissue of the limbs and retroperitoneum in adults. With the exception of primary lung cancer, however, sarcomatous intravascular extension in the lung and infiltration into the left atrium are rare. We report a case of overt left upper lobe pulmonary edema observed on chest radiograms and a mass occupying much of the left atrium and exhibiting hemodynamics similar to those of mitral valve stenosis on transthoracic echocardiography. Intraoperative and pathologic examination showed the mass to be a primary malignant fibrous histiocytoma extending from the left upper pulmonary vein into the left atrium.