Leiomyosarcomas are extremely rare tumors which develop from smooth muscle, usually in the esophagus and large vessels (inferior vena cava, pulmonary artery, and superior vena cava). In very rare cases, leiomyosarcomas develop from small vessels in the soft tissue of the mediastinum. Clinical expression of mediastinal leiomyosarcomas (dysphagia, dysphonia) is related to their large size and the subsequent compression of mediastinal structures. At pathology examination, the gross aspect is one of a single cell tumor. Microscopically, the tumor may be highly undifferentiated making it necessary to use specific immune markers (actin and desmin) or ultra-structural analysis to establish the diagnosis. Treatment of localized tumors is based on surgical excision, either alone or in combination with radiotherapy of the mediastinum. Chemotherapy, generally dexorubicin, is indicated in case of metastatic dissemination, but outcome remains uncertain. As for all soft tissue sarcomas, the prognosis of mediastinal leiomyosarcoma depends on the size of the tumor, its histological structure and its resectability.