Abstract. Thymomas are the most common adult tumors in the anterior mediastinal compartment, and a significant amount of thymomas are complicated by myasthenia gravis (MG). Extended thymectomy (ET) is the primary treatment method for thymomas and is used to completely resect possible ectopic thymus to avoid recurrence. Studies on the effect of MG in thymoma patients following ET are limited. The aim of the present study was to determine whether the presence of MG affects the prognosis of patients with thymoma. The present study consisted of 104 patients with thymoma that underwent ET; 61 men (58.7%) and 43 women (41.3%) (mean age, 54.6 years). In total, 38 patients had MG (36.5%). MG was most frequently observed in World Health Organization (WHO) classification type B2 thymoma compared with other types of thymoma. During the 5-year follow-up period, 11 patients succumbed to a recurrence of thymoma or respiratory failure due to MG. The overall 5-year survival rate in patients without MG or with MG was 89.1 and 76.0%, respectively. The overall survival (OS) rate in patients with Masaoka stages I + II and III + IV was 90.0 and 68.0%, respectively. The OS rate in patients with WHO type A + AB + B1 and type B2 + B3 was 96.9 and 76.8%, respectively. The patients with MG (P=0.026), Masaoka stages III + IV (P= 0.008) and WHO type B2 + B3 (P=0.032) had a poorer prognosis compared with patients without these characteristics. Furthermore, multivariate analysis by Cox regression revealed that age [P=0.032; relative risk (RR)=1.097; 95% confidence interval (CI)=1. 097-1.192] and MG (P= 0.042; RR= 0.167; 95% CI= 0.037-0.940) significantly affected OS rate. In summary, ET is a reliable method for the treatment of thymoma. Long-term survival is expected for patients at early Masaoka stages, and for patients without MG. The prognosis of patients with thymomas with MG is poorer compared with patients without MG. The present findings provide useful information for the future management of patients with thymomas. IntroductionThymoma is a tumor originating from the epithelial cells of the thymus and is a relatively rare neoplasm with an incidence of 0.13 cases per 100,000 individuals per year (1,2). The prognosis of patients with thymoma largely depends on the Masaoka stage of disease and prognosis is poorer for patients with stage III or IV compared with patients with stage I and II tumors (3). The 10-year survival rates are 90, 70, 55 and 35% for stages I, II, III, and IV thymoma, respectively (4). Surgical resection is the primary treatment method for thymoma and a sternotomy is the optimal surgical procedure for thymoma, since it may not be possible to perform a complete thymectomy via thoracotomy on locally advanced lesions, particularly those that are stage III-IV (5,6). Normally, thymoma is a slow-growing tumor, but 40% of thymomas exhibit a locally invasive growth pattern. In addition, thymomas often result in the development of pleural dissemination and distant metastasis (7,8). Therefore, complete resection should be th...