Discussed is a 34-year-old man who hospitalized for manifesting the superior vena cava syndrome. Chest radiograph and chest CT scan revealed a gross tumor shadow in the superior mediastinum and a tiny shadow in the right lung. Superior vena cavography showed intimal stenosis, irregularities, and stiffness of the superior vena cava. Thus, radiotherapy applied to primary lesion of the mediastinum was provided to relieve his complaints. One year later, predominant metastatic pulmonary lesions and skin lesions developed. Light and electron microscopic findings of a biopsied specimen of the skin lesion were consistent with leiomyosarcoma. CYVADIC combination chemotherapy reduced the pulmonary metastatic lesions dramatically. The literature of mediastinal leiomyosarcoma is reviewed and the origin of this tumor is discussed.