Tailgut cyst (TGC), also called retrorectal hamartoma, is a rare congenital lesion arising from persistent remnants of the postanal gut. Malignant transformation of TGC is exceedingly uncommon. We report herein the clinicopathologic features and the follow-up of a new case of a TGC with adenocarcinomatous transformation occurring in a 61 year-old woman.