Of 134 patients with thymoma, 79 (59%) also had myasthenia gravis (MG). The thymoma with MG differed from that in the absence of MG in the following aspects: The ratio of lymphocytes to epithelial cells in the tumor was larger, the polygonal cell type was more prominent (i.e., in 83% of the MG patients this cell type predominated), and the differentiation of epithelial cells in the tumor was more advanced in the group with MG than in the group without MG. Furthermore, the clinical stage of thymoma was earlier, the recurrence rate was lower, and the survival curve was better in the group with MG than in the group without MG. These findings suggest that the thymoma with MG is less malignant than that without MG.