From June 1989 to May 1994, 23 patients with myasthenia gravis underwent thymectomy in the Department of Thoracic Surgery, Asan Medical Center. For the evaluation of the effect of thymectomy, clinical analysis including age and sex distribution, preoperative disease severity, duration of the symptoms, thymic pathology, and postoperative clinical improvement was performed. The mean follow-up period was 15 mon hs(range : 5∼60mon1hs). The effectiveness of thymectomy according to the postoperative symptoms and drug dose was graded as follow : a) complete remission in 5 cases (21.7%), b) clinical improvement with dose reduction in II cases (47.9%), c) clinical improvement without dose reduction in 3 cases(13.0%), 4) unchanged in 2 cases (8.8%), e) early death in 1 case(4.3%) and f) recurrence in 1 case(4.3%). According to the preoperative disease stage by Modified Osserman classification, 6(66.7%) of the 9 patients in stage I and 13 (92.9%) of 14 patients in stage II were clinically improved after thymectomy. although there was no significant statistical differences between these two groups (p> 0.05). According to the thymic pathology, 4 (100%) of 4 cases with normal thymus were clinically improved. Ten (90.9%) of 11 cases with thymic hyperplasia and 5 (62.5%) of 8 cases with thymoma were clinically improved, but there was no significant statistical differences in these three groups (p> 0.05 . The thymectomy resulted in dramatic clinical improvement in 19 (82.5%) out of 23 patients. According to the preoperative disease stage by Modified Osserman classification or the thymic pathology, there was no significant statistical differeneces in the effect of thyinectomy (p> 0.05).