Type AB thymoma with spindle cells arranged in short fascicles (type A component) coursing around lymphocyte-rich nodules (type B component). The spindle epithelial cells are small, plump, oval cells with round or oval pale-staining nuclei containing dispersed chromatin and inconspicuous nucleoli. Mitotic activity is not increased. This tumor was removed from the anterior mediastinum of a 50 y/o female who presented with symptoms of myasthenia gravis. The tumor was grossly encapsulated but showed microscopic infiltration of the capsule (Stage IIa). Thymomas are staged according to the Masaoka-Koga Classification System which combines histologic criteria with elements of the TNM classification. Stage I: Completely encapsulated grossly and microscopically. Stage II: Microscopic capsular invasion (Stage IIa) OR macroscopic invasion into thymic or surrounding adipose tissue, or gross adhesions to but no invasion of pleura or pericardium (Stage IIb). Stage III: Macroscopic invasion into surrounding structures (lung, pericardium, great vessels). Stage IV: Pleural or pericardial metastases (Stage IVa) OR distant metastases via lymphoid or hematogenous route (Stage IVb).