A 38-year-old woman presented with facial edema with neck vein engorgement for about 45 days. Chest roentgenography showed bulging soft tissue opacities in the right superoanterior mediastinum and a lobulated intraluminal mass was noted in the superior vena cava on the venacavogram. The superior vena cava was incised and the tumor located from the junction of the superior vena cava and internal jugular vein to the right atrial inlet was excised. Grossly, the tumor was myxoid or gelatinous in appearance. A combination of microscopic and immunohistochemical features showed myxoid leiomyosarcoma arising from the wall of the superior vena cava.