Of 49 cases of synovial sarcoma, which represent 5.8% of all soft tissue sarcomas with confirmed diagnosis in the files of the Kiel Pediatric Tumor Registry (Kiel, Germany), 35 occurred in patients up to the age of 18 years. The lower extremities were the most common. The 35 cases included 21 biphasic and 14 monophasic fibrous synovial sarcomas. The different cell types constituting synovial sarcoma could be demonstrated by conventional light microscopic study, but more readily so by immunohistochemical study, particularly when antibodies against cytoskeletal components were applied. Aberrant antigen expression was noticed for the neural markers, protein S-100, and neuron-specific enolase. Moreover, four tumors were positive for Ki M7. Collagen type IV was found in all tumors tested. For the 20 patients enrolled in the Cooperative Soft Tissue Sarcoma Study of the German Society of Pediatric Oncology (GPO) the survival rate at 7 years is 63%. When five patients with initial recurrence are excluded, the survival rate is 72%. It is concluded that immunohistochemical study is useful in the diagnosis and differential diagnosis of synovial sarcomas despite certain limitations. Multimodality treatment approach has improved the overall prognosis. There is no relationship between histologic subtype and prognosis according to the classification scheme employed in this study. Cancer 67:1667Cancer 67: -1672Cancer 67: ,1991. . YNOVIAL SARCOMA is a malignant soft tissue neo-S plasm which primarily occurs in close association with tendon sheaths, bursae, and joint capsules. In the past, because of this anatomic relationship, its origin has been linked to synovial tissue.' However, several u l t r a s t r ~c t u r a l ~~~ and immunohistochemical have failed to demonstrate a synovial origin. It is generally accepted today that synovial sarcoma is probably derived from a primitive mesenchymal precursor cell6 with the capability of differentiation toward epithelial-like and fibroblast-like cells, hence the proposal by Miettinen and From the