A biphasic synovial sarcoma occurring in the anterior and inferior mediastinum in a 19-year-old woman is reported. A biopsy showed a mesenchymal proliferation and the tumor was first misdiagnosed as a hemangiopericytoma. Inefficacy of chemotherapy led to a tumorectomy. Histologic and immunohistochemical studies on multiple samples showed a biphasic tumor. Ultrastructural study confirmed the presence of epithelial elements and cytogenetic analysis disclosed a translocation t(X;18) (p11;q11), leading to a diagnosis of synovial sarcoma. Synovial sarcoma of the mediastinum is very rare and to our knowledge has not been previously studied with the help of cytogenetics. Given the biphasic pattern of the tumor and its mediastinal location, it can be confused with mesothelioma. This stresses the interest of chromosome analysis in the study of tumors histologically difficult to classify.