A rare case of retroperitoneal sarcoma arising from an aneurysm sac is diagnosed in a patient following tube graft repair of the abdominal aortic aneurysm. The radiographic studies illustrate the characteristics of malignancy which are distinguishable from more common vascular complications. Malignancy should be considered in the differential diagnosis of retroperitoneal masses during radiographic follow-up of patients after aortic surgery. Sarcoma is optimally treated with complete excision, sometimes requiring concomitant vascular reconstruction. Radical en bloc resection was not attempted in this case due to extensive involvement of the aorta and inferior vena cava.