A 23-year-old woman with myasthenia gravis underwent thymectomy in September, 1975 by a trans-sternal approach for the removal of an hyperplastic gland. Surgical intervention was followed by marked improvement of her symptoms. Five years later, symptoms recurred. In March, 1982 diagnostic pneumomediastinum disclosed an anterosuperior mediastinal mass which, on excision, proved to be an hyperplastic thymus. This report confirms previous findings of thymus regrowth after surgery. It also demonstrates that surgical techniques using the trans-sternal approach do not guarantee complete removal of the thymus.