Occurrence of thymoma after an extended thymectomy through a full median sternotomy for nonthymomatous thymectomy has been very rarely reported. A 60-year-old male patient who had myasthenia gravis (MG) for 11 years had an extended thymectomy operation with a pathology of thymic hyperplasia and developed a mass in the aortopulmonary window. We resected the mass via anterior left thoracotomy by dividing the ductus arteriosus and mobilizing the aorta. Cord vocal augmentation procedure was done due to the resection of the left recurrent laryngeal nerve.