-Myasthenia gravis (MG) is an autoimmune disease characterized by fluctuating muscle weakness, caused by impaired neuromuscular transmission. Patients with MG can present other autoimmune diseases in association, commonly hypo or hyperthyroidism. The association of MG to demyelinating disease is rare and has been described before. We report on three Brazilian patients with MG that presented distinct demyelinating diseases, two monophasic and one recurrent neuromyelitis optica, several years after the diagnosis of MG, and discuss their clinical courses.Key Words: myasthenia gravis, demyelinating disease, association. Doenças desmielinizantes em pacientes com miastenia gravisResumo -Miastenia gravis (MG) é doença autoimune caracterizada por episódios de fraqueza muscular alternados com melhora, causada por bloqueio da junção neuromuscular. Pacientes com MG podem apresentar outras doenças autoimunes, comumente hipo ou hipertiroidismo, e a associação de MG com doenças desmielinizantes é raramente descrita. relatamos três pacientes brasileiros com MG que desenvolveram doenças desmielinizantes, dois monofásicos e um neuromielite óptica recorrente, vários anos após o diagnóstico de MG e discutimos seus cursos clínicos.PAlAvrAs-chAve: miastenia gravis, doenças desmielinizante, associação. Myasthenia gravis (MG) is an autoimmune disease characterized by fluctuating muscle weakness, caused by impaired neuromuscular transmission. Autoantibodies specific for the human nicotinic acetylcholine receptor are present in 70 to 80% of the patients, and the remaining cases can be associated with antibodies targeted at muscle specific kinase (MusK) and other proteins in the post-synaptic membrane 1,2 . MG has a bimodal incidence: (1) 20 to 40 years-old, predominated by women, and (2) 60 to 80 years-old, predominated by men 2 .Patients with MG commonly have thyroid disease and can present with nonspecific immune system abnormalities, such as positive auto antibodies and alopecia areata 3-5 . rarely, MG can be part of multiple autoimmune syndromes, including autoimmune diabetes mellitus, thrombotic thrombocytopenic purpura, sjögren syndrome, systemic lupus erythematosus, vitiligo, among other rare diseases 5 . In the past 20 years there has been an increasing number of reports on patients with MG presenting demyelinating diseases (dd) 6-15 , including multiple sclerosis, neuromyelitis optica (NMo), transverse myelitis and optic neuritis. however, it is not known whether this association is also part of unspecific immune activation, genetic susceptibility or if it just happens by random.We herein describe three Brazilian patients with MG that presented different dd and discuss their clinical courses. methoDWe reviewed the records of 630 patients followed for MG at the Neuroimmunology clinic of the Federal University of sao Paulo between 1979 and 2006. The diagnosis of MG was based on clinical history, neurological examination, electrophysiological testing and response to acetyl cholinesterase inhibitor medication (pyridostigmi...