Full clinicopathological details and clinical follow up of a case of malignant transformation within a tailgut cyst are presented. A 43 year old woman presented with signs and symptoms of an imminent threatened abortion. Routine examination identified a coincidental, asymptomatic retrorectal/presacral mass. Following imaging studies, surgical resection was carried out and an adenocarcinoma arising within a pre-existent tailgut cyst was identified by microscopy. Four years later the patient presented with neurological symptoms consistent with local recurrence of the tumour. Surgical biopsies confirmed this diagnosis and she was subsequently started on chemotherapy. She died soon after from a cause unrelated to the disease, after declining further active intervention. Differential diagnosis of such cases includes (cystic) teratoma, epidermal cyst, rectal duplication cyst, anal gland cyst and carcinoma, extension of local carcinoma, and metastatic disease. It is recommended that these lesions be completely excised when detected incidentally.