This report documents the clinical k pathologic features of 33 patients treated for thymoma for 11 years & 6 months between September 1977 and February 1989. At the Thoracic & Cardiovascular Surgery, Yonsei University, College of Medicine of the group, 31 patients treated with surgery were examined for the result of operation & prognosis. Mean age was 50 years. Thirteen were female and twenty were male. Dyspnea on exertion and chest discomfort were common in the patients without myasthenia gravis. Fourteen patients[42.6%] had myasthenia gravis and one patient had autoimmune thyroid disease. Four patients[12.1%] presented without symptoms attributable to their thymoma. Histologic review disclosed 12[36.4%] epithelial thymoma, 10[30.3%] mixed lymphoepithelial, 9[27.3%] lymphocytic, 1[3.0%] spindle cell and 1[3.0%] unknown cell thymoma. They were classified according to Masaoka`s clinical staging criteria; by these criteria, 8 patients were stage I, 5 patients were stage II, 15 patients were stage III and 3 patients were stage IV. Total excision of mass was possible in 20 patients. Partial excision of mass in 4 patients and biopsy in 7 patients were carried out during the operation. There was only one operation mortality. Follow-up was possible in 26 patients and follow-up ranged from 4 months to 10.5 years[mean 28.9 months]. One-year survival rates were 77.8% and eight patients expired during follow-up period. Eleven[78.6%] patients with myasthenia gravis were improved after the operation. This observation suggests that the most significant factor determining the survival is whether or not total surgical excision had been performed.