The term myxosarcoma is currently not used in standard classification for soft tissue tumors, but restricted to cardiac tumors. Primary cardiac myxosarcoma is a very rare disease and is difficult to differentiate from myxoma clinically and pathologically. We report a case of left atrial myxosarcoma with widespread systemic metastasis in a 21-yr-old male. The patient presented with sudden onset of intermittent dyspnea and orthopnea. Echocardiography showed a mobile, pedunculated tumor, 7.5×5×2 cm in size, at left atrium. Histologically, the excised tumor showed an amorphous finely fibrillar and mucinous stroma, in which irregular cords and clusters of lepidic cells and large stellate cells with plump vesicular nuclei resembled the usual type of cardiac myxoma were noted. And it showed focally cellular area with great nuclear pleomorphism and frequent mitoses. The patient received combination chemotherapy, peripheral blood stem cell collection transplantation and operations for systemic metastases in the brain, skeletal muscle and lung. He is alive at present 37 months after initial diagnosis and has no more new metastatic lesion.