A 39-year-old woman who had experienced slight chest discomfort for a few months was referred to our hospital with an abnormal shadow on a regular checkup chest X-ray film. A computed tomography scan revealed a large well-defined mass in close relationship to the great vessels of the anterior mediastinum. Magnetic resonance imaging showed the anterior mediastinal mass, which was about 25 cm in diameter, expanding to the left pleural cavity with heterogeneous intensity. Because of the size and location of the mass, a combination of anterolateral thoracotomy and partial longitudinal median sternotomy--so-called hemiclamshell incision--was chosen, allowing excellent visualization and complete dissection of the giant tumor. The final histopathology of the resected specimen confirmed well-differentiated liposarcoma.