Thymoma is defined as a neoplasm of the epithelialreticular framework cells of the thymus. The treatment of choice and the prognosis for patients with thymoma are still controversial subjects due to lack of a uniform histological classification and standardized criteria for typing thymic tumors. Between June 1985 and May 1989, eight patients underwent thymomectomy at the Department of Thoracic and Cardiovascular Surgery, Korea University Medical Center. A clinical analysis was performed and the following result was obtained. In histologic cell type, epithelial type was 2 cases, lymphocytic type 3 cases, and mixed type was 3 cases. The clinical stage was classified by Masaoka’s classification that Stage I was 4 cases, Stage II; 2 cases, Stage III; 2 cases and Stage IV was none. Myasthenia Gravis was associated with thymoma in 5 patients. In 8 patients, thymomectomy were performed. Adjuvant radiotherapy was performed in one patient with Stage I due to suspicious tumor remnant and in Stage II, III patients routinely. Combined chemotherapy was performed in one patient with Stage III due to local recurrence. We concluded that the most important prognostic factor of thymoma is extent of tumor invasion. And the presence of Myasthenia Gravis is no more prognostic factor. Surgical resection of thymoma is treatment of choice, If local invasion or distant dissemination is present, postoperative radiotherapy and /or combined chemotherapy is recommended.