Thymectomy and subsequent short course of corticotropin in high doses did not change the course of persistent and unusually severe congenital myasthenia gravis in a 5-year-old boy. Grossly, the thymus appeared normal in size and consistency for the patient's age. On histopathologic examination several changes consistent with an autoimmune disorder were demonstrable, including lymphoid hyperplasia and large numbers of thymic plasma cells. Electron microscopic studies showed intralobular vascular changes and the presence of thymic myoid cells. Their possible significance is discussed.