A 17-year-old female developed myasthenia gravis (MG) and retroperitoneal giant lymph node hyperplasia (GLNH). Her myasthenic symptoms improved after the removal of the retroperitoneal tumor. Moreover, she has done well in daily activities without medication for one year. To our knowledge, the coexistence of MG and GLNH has been reported in only one case. We encountered this second case with this association and believe it has some relationship with the first. Further research on this patient is warranted to elucidate the problem.