Primary mediastinal liposarcoma was observed in a 73-year-old man. Because of tight adhesions to adjacent tissues, neither complete resection nor surgical debulking of the tumor was possible. A T-tube was inserted into the patient's trachea for severe dyspnea, and he was treated with radiotherapy and an oral peroxisome proliferator-activated receptor-gamma agonist. The patient died 6 years after the initial diagnosis. Autopsy revealed liposarcoma composed of 3 subtypes in the primary tumor: well-differentiated, dedifferentiated, and round cell components. Round cell and dedifferentiated liposarcomas were predominantly observed in the metastatic nodules.