Retrorectal, developmental tail gut cysts, include dermoid cysts, rectal duplication cysts and retrorectal cyst-hamartomas. Retrorectal cyst-hamartomas (RCH) are derived from remnants of the tail gut, the most caudal part of the embryonic hind gut, which normally involutes by the 8(th) wk of embryonic development (3-8 mm stage). They have specific radiological and histopathological features that distinguish them from other similar formations (dermoid cysts, enteric duplication cysts and teratomas). We report a patient with adenosquamous carcinoma arising within RCH, who underwent complete resection of the cyst through anterior laparotomy, and reached complete (recurrence-free for 14 mo, so far) functional recovery. The cyst was incidentally discovered during hysterectomy 12 years ago. Diagnostic, therapeutic and histopathological aspects of this rare case are discussed. The mentioned period between diagnosis and surgical treatment suggests that RCH, given enough time, can develop malignant degeneration, and should be resected at the time of diagnosis.