Authors studied 123 patients with myasthia gravis, who were admitted to the Seoul National University Hospital from 1981 to 1986 to get the incidence of thymoma in myasthenia gravis. 1. Among 66 cases, 23 were said to have the thymoma by radiological studies such as chest CT scan. 2. True thymoma were confirmed by histopathologic study in 10 out of 15 cases of CT-made thymoma. 3. The incidence of the associated thymoma in myasthenia gravis was calculated to be 19.2%. 4. Among 10 cases of thymoma, 7 cases were above 40 year of age, 3 cases below 40. 5. When these were classified by modified Osserman's classification, type IIA was numbered in only one, type IIB in 5, type III in 2, and type IV in 2. Therefore it is concluded that the thymoma plays significant role in myasthenia gravis and it is important to manage it in the diagnosis and treatment of myasthenia gravis.