Myasthenia gravis (MG) is an autoimmune disorder characterized by autoantibodies against acetylcholine receptors. MG is generally an isolated disorder but may occur concomitantly with other autoimmune diseases. We describe an eighteen-year-old girl with MG who was admitted to our clinic with secondary amenorrhea and diagnosed as autoimmune oophoritis. Since her myasthenic symptoms did not resolve with anticholinesterase therapy, thymectomy was performed. After thymectomy, her menses have been regular without any hormonal replacement therapy. To our knowledge, this is the first report on a patient with autoimmune ovarian insufficiency and MG in whom premature ovarian insufficiency resolved after thymectomy, without hormonal therapy.