We report two female patients with neuromyelitis optica (NMO, Devic's syndrome) following or coincidental with myasthenia gravis (MG). Their illnesses were characterized by subacute myelitis with optic neuritis, high serum levels of muscle acetylcholine receptor antibody, and autoimmune thyroid disease. Both patients fulfilled the clinical criteria of NMO, however, NMO-IgG, autoantibody against aquaporin-4 water channel, was absent from their sera. Both NMO and MG are relatively rare diseases. The considerable coincidence of these two disorders suggests that there is a subgroup of patients with NMO having a common immunological pathogenesis with MG.