A retrospective study of 56 patients with synovial sarcoma was done to search for possible prognostic factors affecting survival. In a univariate analysis, age (≥20 years), site (proximal), large tumor size (≥5 cm), mitotic rate of more than 15 per 10 per high-power fields (hpf), high nuclear grade, tumor necrosis of more than 50%, the presence of rhabdoid cells, a small number of mast cells (<20/10 hpf), and a high stage according to the criteria of the American Joint Committee (AJC) staging of soft tissue sarcoma, significantly decreased survival (log-rank test; p < 0.05). Moreover, when comparing the 10-year survival (11 patients) and the deaths within 1 year (12 patients), size, high nuclear grade, presence of rhabdoid cells, tumor necrosis, and stage were all considered to be significant prognostic factors (two-sided chi-square test; p < 0.01). In a multivariate analysis (Cox's model), stage was the only strong predictor of a poor prognosis (p = 0.011). We therefore concluded that large tumor size, high nuclear grade, the presence of rhabdoid cells, extensive tumor necrosis, and a high AJC stage were adverse prognostic factors in synovial sarcoma.