The case was a 69-year-old female who was referred to this department because of a shadow on chest x-ray while receiving dialysis twice in a week for chronic renal failure. She had a quickly growing large mass at the posterior mediastinum observed by the simple chest x-ray film, CT and MRI before the operation. As a result of biopsy, the mass was found to be a mesenchymal tumor and diagnosed as sarcoma originating at the posterior mediastinum. Operation revealed continuity of the mass to the esophagus with ambiguous demarcation although there was only a slight adhesion involving a part of the lower lobe of the lung, aorta and diaphragm. Therefore, the mass was considered to be a leiomyosarcoma originating from the esophagus, and tumor enucleation was performed. The tumor size was 13.5×13.5×9.5cm, weighing 840 g with hemorrhagic and necrotic foci within the mass. The tumor was diagnosed as GIST (Gastro-Intestinal-Stromal-Tumor) of a mesenchymal tumor originating in the esophagus.GIST originating in the esophagus is often difficult for the differential diagnosis from posterior mediastinal tumor for the mode of growth. Therefore, careful preoperative diagnosis is required.