We report the case of a 60-year-old woman with myasthenia gravis (MG) and Basedow's disease who seven years after thymectomy developed subacute myelitis, a limited form of neuromyelitis optica (NMO). The patient presented with a centrally located long spinal cord lesion (LCL) on cervical cord MRI, anti-aquaporin 4 (AQP4) antibody in serum, and HLA DPB1*0501. Brain MRI showed no specific findings of classic multiple sclerosis (MS). CSF study showed elevated protein (67 mg/dl) but a normal IgG index (0.63) and no oligoclonal IgG bands. After repeated methylprednisolone pulse and immunoadsorption therapies, the T2-high signal lesion shrunk and tetraparesis improved. We reviewed the English and Japanese literature and found reports of 30 patients showing MS including NMO complicated with MG; 27 had been diagnosed as MS after thymectomy. Among these 27, 16 of 17 who were examined by spinal cord MRI and for anti-AQP4 antibodies were NMO. Only one patient with signs and symptoms localized to the optic nerves and spinal cord showed no LCL and was not examined for anti-AQP4 antibodies. In autoimmune disorders of the central nervous system after thymectomy in patients with MG, NMO is more predominant than classic MS.