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Background: Chronic graft-versus-host disease (GVHD) appears several months following allogenic hematopoietic stem cell transplantation (HSCT) and is clinically analogous to autoimmune disorder. Polymyositis is a common neuromuscular disorder in chronic GVHD, but myasthenia gravis (MG) is extremely rare. Hence, its pathophysiology and treatment have not been elucidated.Case Presentation: A 63-year-old man with a history of chronic GVHD presented with ptosis, dropped head, and dyspnea on exertion, which had worsened over the previous several months. He showed progressive decrement of compound muscle action potential in the deltoid muscle evoked by 3-Hz repetitive nerve stimulation, a positive edrophonium test, and elevated levels of serum anti-acetylcholine receptor antibodies, which suggested a diagnosis of generalized MG. No thymoma was found. Flow cytometric analysis revealed a remarkable depletion of peripheral Tregs (CD4+CD25highFOXP3+ cells, 0.24% of the total lymphocytes). Administration of prednisolone and tacrolimus was insufficient to alleviate his symptoms; however, the use of rituximab successfully improved his condition.Conclusions: Myasthenic symptoms appeared in the process of tapering prednisolone for the treatment of chronic GVHD, supporting the diagnosis of MG associated with chronic GVHD. The present case proposes a possibility that reduction of Tregs might contribute to the pathogenesis of MG underlying chronic GVHD. Immunotherapy with rituximab is beneficial for treatment of refractory MG and GVHD. [ABSTRACT FROM AUTHOR]