A 37-year-old Japanese man with chronic myeloid leukemia (CML) developed myasthenia gravis 29 months after bone marrow transplantation (BMT) from an HLA one locus-mismatched brother. Proximal muscle weakness and bilateral ptosis occurred along with the exacerbation of chronic GVHD shortly after sudden cessation of cyclosporine (CYA) and prednisolone. The diagnosis of myasthenia gravis was made based on clinical symptoms and elevation of an anti-acetylcholine receptor antibody titer and all symptoms related to myasthenia gravis promptly diminished with the start of treatment for chronic GVHD. In most previously reported cases, the underlying disease was aplastic anemia (6 of 7 cases) and donors were of the opposite sex (6 of 7 cases). The haplotypes HLA B7 (3 of 5 cases), B35 (3 of 5 cases), and DR2 (3 of 3 cases) were common. All cases suffered from chronic GVHD. The present case had only chronic GVHD and HLA B7 as a background for myasthenia gravis after BMT. The abrupt cessation of immunosuppressive therapy may also be related to the development of myasthenia gravis after BMT.