This study sought to define predictors of recurrence after resection of thymic tumors. A single-institution retrospective study was performed of 179 patients who underwent resection of a thymic tumor from 1972 through 2003. Resection was complete in 90% (161/179) of patients. After a median follow-up of 115 months, the recurrence rate was 11% (20/179), the tumor-related death rate was 7.8% (14/179), and the overall death rate was 36.3% (65/179). Tumor recurrence correlated with advanced stage and histology ( P < .0001). The difference in recurrence between Masaoka stage I (0) and II (1.7% 1/59) was insignificant. Recurrence rates correlated with World Health Organization tumor type: A and AB, 0%; B1 and B2, 8% (4/51); B3, 27% (14/51); and C, 50% (2/4; P < .0001). Tumor size separation into quintiles demonstrated a step-up of recurrence at 8 cm (<8 cm, 1.8% 2/113; ≥8 cm, 28% 18/64; P < .003). Multivariate Cox modeling demonstrated that Masaoka stage (odds ratio, 5.70; P < .001), World Health Organization histology (odds ratio, 5.77; P = .003), and size (odds ratio, 1.16; P = .001) were independent predictors of recurrence. The Masaoka staging system could be collapsed to 3 degrees of invasion by combining stages I and II. The World Health Organization histologic type can be simplified for clinical use into A (A, AB), early B (B1, B2), advanced B (B3), and C tumors. Size of 8 cm or larger is an independent risk factor, even when patients with Masaoka stage III tumors are considered alone, and might identify candidates for preoperative therapy.