One hundred and fifty-eight cases of mediastinal tumors have been studied clinicopathologically, based on patients of Nagasaki University Hospital from 1961 to 1984. Of these tumors, 29.7% were thymic tumors, 23.4% neurogenic tumors, and 21.5% were germ cell tumors, the frequencies of which reflected the same tendency as other such reports in Japan. Thirty-three cases were malignant tumors. Among them, thymomas were the most common (48.5%) as Wada et al. have reported. Extrathoracic metastases have observed in 2 thymomas. Thymomas associated with myasthenia gravis were seen in 6 cases in which 5 patients are still alive.