To the best of our knowledge this is the seventh case report of monozygotic twins both affected by myasthenia gravis (MG). The monozygotism was proven by sex identity, blood group and HLA determinations ('O' Rh +, A2, A19.2, B40, CW3). One paternal aunt was also affected and the three cases have high titles of anti-acetylcholine receptor antibodies and anti-striated muscle antibodies, which indicate an acquired form of MG. After several myasthenic crises the twins are now doing well with corticosteroid therapy. The paternal aunt has a more benign form of MG, with ocular and limb involvement (grade IIa of Osserman) and was submitted to thymectomy. The authors discuss the use of corticosteroid for the infantile form of MG instead of thymectomy, based on the immaturity of the immune system in the young age. The use of other immunosupressive drugs is not advisable because of potential hazardous development of neoplasms.