Myasthenia gravis is a disorder of neuromuscular transmission which is characterized fatigue and weakness of the voluntary muscle. From 1985. 1 to 1991.6, 34 patients underwent managed including 12 patient thymectomy, at the kyunghee medical center. A clinical analysis was performed and following result was obtained. Among the 34 patients, male to female ratio was 11 : 23 and the age was ranged from 2 years to 63 years. Thymectomy was done in 12 case and 1 case of malignant thymoma was not resectable. There were 2 deaths after operation due to respiratory failure and 1 death dur to gradually progression. THe effect of thymectomy in mtasthenia graves was 66.7% (8cases). The prognostic factors were severity of the disease and histopathological findings. Others were not related to the prognosis of the mayasthenia gravis. The severity of the myasthenia gravis was classified by Osserman modified classification.