The relationship between prognosis of the disease and the type of surgery were reviewed in 110 cases of thymoma. Surgical procedures were classified into four types: total resection (TR), subtotal resection (SR), partial resection (PR) and exploratory thoracotomy (ET), and the extent of the disease was expressed as stage I through IV based on Masaoka's classification. Ten-year survival rates for stage III and IVa diseases were 5 6.2% in TR, 37.9% in SR and 30.3% in PR. None of the patients in ET survived more than 28 months. Tumor death after surgery was not observed in TR. Two of three patients with stage IVa disease who underwent TR are alive without recurrence of the tumor at 37 months and 6 months after surgery. The pleuropneumonectomy was performed in the former case. One of four patients who underwent SR and one of three patients who underwent PR both with stage IVa disease are alive at 71 and 150 months after surgical intervention, respectively, in spite of the presence of the tumor relapse. In these patients, repeated chemotherapy and radiotherapy were administered following the resection of the primary lesion. In conclusion, it was indicated that the improvement of the survival rate could be obtained by resection the tumor as completely as possible in stage III and IVa thymoma, as long-term survival has been proved to be possible by macroscopically complete removal of the primary site and pleural disseminations followed by combined adjuvant therapy in patient with stage IVa disease.