Description of a case of leiomyosarcoma of pericardium in a 44 years old woman. Clinically, the most important symptoms were edema and cyanosis of the face, neck, arms and of the upper half of the body, due to thrombosis of superior vena cava. Because of these vascular signs, a mediastinal tumor was suspected. The autopsy revealed that the enlargement of the heart showed by XR examination was due to a neoplastic invasion of the pericardial cavity. The heart was not involved and there was no metastasis. The thrombosis of superior vena cava was due to the compression by the tumor. The histologic examination proved, without any doubt, that the tumor was a leiomyosarcoma.