The authors studied 20 cases of myasthenia gravis with radiological examination showing the persistence or hipertrophy of the thymus. In these cases were made radiological examination without contrast and with mediastinal emphysem. The persistent of hypertrophy thymus is proved by this last method in almos all cases. The several radiological aspects of the thymus were analysed and its relations with the therapeutic by ACTH, radiotherapy and/or thymectomy presented; however, in this field, the authors can't be conclusive because they have still a few cases.