Three hundred seven cases of patients who underwent operation for thymoma (196 of whom had myasthenia gravis) were analyzed to assess the prognostic values of Masaoka clinical staging, completeness of resection, histologic classification, history of myasthenia gravis, and postoperative radiotherapy. According to the Masaoka staging system, 135 thymomas were stage I, 70 were stage II, 83 were stage III, and 19 were stage IV. According to the Verley and Hollmann histologic classification system, 67 thymomas were type 1, 77 were type 2, 139 were type 3, and 24 were type 4. Two hundred sixty patients underwent complete resection, 30 underwent incomplete resection, and 17 underwent biopsy. Postoperative radiotherapy was performed mainly in cases of invasive or metastatic thymoma. Mean follow-up was 8 years; eight patients were unavailable for follow-up. The overall 10- and 15-year survivals were 67% and 57%, respectively. In univariate analysis, three prognostic factors were established: completeness of resection, Masaoka clinical staging, and histologic classification. Furthermore, among patients with stage III thymomas, survival was significantly higher for patients with complete resection than for patients with incomplete resection (p < 0.001). Completeness of resection should therefore be taken into account in clinical-pathologic staging. We did not find any significant difference with respect to disease-free survival between patients who had postoperative radiotherapy and those who did not. In multivariate analysis, the sole significant prognostic factor was completeness of resection. On the basis of these findings, a new clinical-pathologic staging system is proposed.