Résumé : Tailgut cysts are rare benign cystic lesions of the retrorectal space. The recommended treatment is complete resection because these cysts have occasionally shown malignant transformation. However, a high index of clinical suspicion is required to reach a diagnosis. We report a 68-year-old man complaining of a subcutaneous mass in his right buttock. Magnetic resonance imaging (MRI) showed a large cystic mass extending 25.7 cm from the pelvis to the buttock. Radiological features indicated a benign cystic tumor, but the level of serum carcinoembryonic antigen (CEA) (87.5 ng/mL) was increased. An incisional biopsy did not define the true histological nature of the lesion and was not useful for surgical planning. Although MRI could not detect malignant changes, the elevated serum CEA indicated malignant degeneration. The patient required a Miles operation for complete resection. Surgical pathology revealed focal areas of high-grade adenomatous and adenocarcinomatous changes in the cyst wall. After surgery, the serum CEA level decreased to below the normal range. The case presented here shows that early malignant degeneration of TGC is difficult to detect by MRI. Thus, a tailgut cyst should be completely removed, even if radiological examination cannot detect malignant features. Measurement of serum CEA may be helpful when the tumor expresses this antigen.