A 21-year-old nulliparous Greek woman complained of chronic noncyclic pelvic pain. Abdominal and vaginal examinations were unremarkable, whereas on rectal examination, a soft extraluminal mass was found posteriorly and left laterally. The rectal mucosa was normal on rigid rectosigmoidoscopy. A pelvic ultrasound scan revealed a cystic lesion posterior to the middle rectum, and blood tests showed a moderately elevated CA 19-9 (79IU/ml), whereas all other tumour markers were normal. Computed tomography (CT) of the whole abdomen excluded other intra-abdominal pathology and provided further information regarding the anatomic relations of the lesion. The cyst lay posterior and left lateral to the middle rectum above the level of the pelvic floor and was contiguous neither to the rectal wall nor to the sacrum. Its maximal diameter was about 7 cm. After administration of preoperative antibiotic prophylaxis, a laparotomy was undertaken through an infra-umbilical midline incision. Moderate bilateral ovarian endometriosis and minor endometriosis of the pelvic peritoneum were found; these were ablated with surgical diathermy. Subsequently, the pelvic peritoneum was opened, and the retrorectal space was carefully dissected to avoid injury of the pelvic nervous plexuses and the hypogastric nerves. The retrorectal cystic lesion was removed intact, and on histologic examination was found to be a suppurated endometrioid cyst. The patient made an uneventful recovery and was discharged on the third postoperative day. The treatment was completed with a six-month course of a gonadotropin-releasing hormone (GnRH) analogue. One year postoperatively, she remained free of symptoms, and follow-up pelvic imaging showed no recurrence of endometriosis.