A 28-year-old male with bilateral testicular seminoma underwent bilateral orchiectomy and radiation therapy of the retroperitoneum. After 17 years, he had a retroperitoneal tumor detected, which was removed 7 years later at age 52 because of its progressive enlargement. Due to its partially cystic and partially solid structure, the radiologic findings could not exclude the possibility of regressively altered seminoma metastasis. After radical surgical removal of the tumor, the histopathological and immunohistochemical examination of the tumor revealed ancient schwannoma. These tumors, although unusual, might pose a clinical diagnostic challenge with the risk of undesired overtreatment.