Unusual retrorectal lesion.

Retrorectal lesions are rare entities. We report the complete clinicopathological details of an unusual retrorectal lesion composed of low-grade endometrioid adenocarcinoma and propose that this is a malignant change in pluripotent cells arising within a long-standing retrorectal tailgut lesion. To the best of our knowledge, this is the first case report of this malignancy in a retrorectal tailgut cyst.