Tailgut cyst are congenital cystic lesion arising from remnant of the embryological postnatal gut. Tailgut cyst are multinodular, uncapsulated and usually well-circumscribed. Presacral cysts are rare in adult and most of the lesions are benign. Malignant degeneration can occur, however is extremely rare.We present the case of a 74 years old woman with slow increase in size and malignant degeneration of a tailgut cyst. Five years before, during the follow up after mastectomy for cancer, she manifested rise of CA 19-9 tumor marker and a presacral cystic collection on thoraco-abdominal CT. She was followed with CT and MRI that showed that the cyst, with a solid component of the wall, was growing larger. After a five-year evolution, the cyst was resected. The histological examination on the solid component demonstrated intestinal adenocarcinoma.MRI ant TC can play essential role in the preoperative detection and characterization for the differential diagnosis, treatment strategies and evaluate neoplastic degeneration. Due to the risk of malignancy surgical resection must be performed after the diagnosis. Surgical therapy is mandatory when the cyst grow larger and a solid component is present.Presacral cyst, Retrorectal tumors, Tailgut cyst.