Myasthenia gravis (MG) is a rare autoimmune disease of the neuromuscular junction. It is characterized by variable weakness and excessive fatigability of skeletal muscles. In the last few years, numerous reports have been published showing the association between autoimmune diseases, such as systemic erythematous lupus or rheumatoid arthritis, with lymphoid neoplasias. The association between MG and lymphoid neoplasia seems to be less frequent. To analyze this association we reviewed the MG patients in the Department of Neurology, Hospital Salvador of Santiago, Chile. During a three-year period we identified four patients who developed different lymphoproliferative disorders: two with B-cell lymphoma, one with chronic lymphocytic leukaemia and one plasmacytoma with an associated amyloidosis. The MG was generalized but mild, all cases classified as type IIa according to the definition proposed by the MG Foundation of America. The neoplasia appeared two to 36 years after the onset of MG. These cases provide additional evidence of the association between MG and lymphoproliferative disorders.