Thymus gland is a kind of endocrine organ which secretes thymosin and thymoprotein. There can be developed variable lesions like thymoma, thymic hyperplasia, thymic cyst, thymolipoma, and carcinoid tumor of the thymus gland. We have experienced 25 patients of thymic disease: thymoma 12, thymic hyperplasia 11, thymic cyst 1, carcinoid tumor 1. The age distribution were ranged from 3 to 66 years and the sex ratio was 1:1.8 [male to female]. Thymectomy was performed in all cases, but 3 cases with deep infiltration to the adjacent structures were not resectable completely. Malignancy [all thymoma] were 5 and the rest were benign. Two cases were died of recurrence after tumor resection. Myasthenia gravis occurred in 10 cases. Among them, 2 were thymoma and 8 were thymic hyperplasia. We could obtained the result that thymectomized cases reached 2 remission and 5 improvement. Myasthenia gravis treated medically [18 cases] had no remission and only 2 clinical improvement. In the light of these results, early radical thymectomy would be most favorable treatment in not only thymic tumor, but generalized myasthenia gravis.