Liposarcoma is rare in children and rarely occurs in the posterior mediastinum in any age group. A massive intrathoracic tumor in a 17-year-old young man was a diagnostic dilemma; preoperative radiographic evaluation and biopsy led us to believe it was a teratoma. At operation, a poorly differentiated myxoid liposarcoma originating from the posterior mediastinum was found and excised. To our knowledge, this is the first liposarcoma of the posterior mediastinum reported in a patient less than 18 years old.