To identify most significant and therapeutically relevant prognostic factors in adults with localized primary synovial sarcomas (SS) and to confirm the usefulness of the French Federation of Cancer Centers (FNCLCC) grading system, the prognostic impact of which has been already proven in soft tissue sarcomas. Data on 128 patients with nonmetastatic SS collected from a cooperative database by the FNCLCC Sarcoma Group between 1980 and 1994 were studied retrospectively. Immunohistochemistry was performed at diagnosis in 77 cases (61%). The tumors were classified as biphasic (n = 45), monophasic fibrous (n = 72), and poorly differentiated (n = 10) subtypes. Histologic grade was determined according to the FNCLCC method, and vascular invasion was assessed in every case. The 5-year disease-specific survival (DSS) rate for this series of patients with localized SS was 62.9% (+/- 9.6% SD) with a median follow-up time of 37 months (range, 8 to 141 months). In multivariate analysis, the adverse risk factors associated with decreased DSS were International Union Against Cancer/American Joint Committee on Cancer stage III/IVA disease, male sex, and truncal tumor locations. For metastasis-free survival (MFS), disease stage III/IVA, tumor necrosis, and monophasic subtypes were the major factors associated with a less favorable prognosis. Separately, when not using disease stage, tumor necrosis, and mitotic activity, histologic grade became the most significant prognostic factor for both DSS and MFS. In addition, larger tumors and older patients become associated with a significantly worse prognosis. Independent adverse risk factors for local recurrence-free survival included histologic grade 3 and truncal tumor location. These data confirm that not all SS present the same severe outcome. High-risk patients identified on the basis of these parameters may qualify for an aggressive treatment approach.