Recently, the prognosis of myasthenia gravis (MG) has improved considerably because the commercial measurement of acetylcholine receptor (AChR) antibody has been introduced and early treatment with steroid at large quantities has been utilized. On the other hand, the efficacy and the safety of thymectomy for MG patients without thymoma are still controversial. A meta-analysis of thymectomy for MG patients without thymoma revealed an uncertain effect. Since 2005, a controlled randomized study for thymectomy in patients with MG has been in progress at 66 centers in 16 countries. This study compares the efficacy and safety of steroid combined with thymectomy versus those of steroid alone. The total number of study patients is 150. When patients are randomized into the group of steroid plus thymectomy, they receive extended trans-sternal thymectomy prior to receiving steroid. In both groups, patients receive oral steroid on alternate days. The dose of steroid increases relatively rapidly, and is then maintained at 1.5mg/kg BW or 100mg on alternate days. After 4 months, the maintained dose of steroid is decreased according to the protocol. This study, initially planned and conducted by Dr. John Newsom-Davis, should provide solid evidence on the efficacy and safety of extended trans-sternal thymectomy for MG without thymoma.