Case presentation A 6-month-old female infant with an absent left thumb ( Figure 1) was referred to a pediatric surgeon because of rectal prolapse. She was the full-term product of an uncomplicated pregnancy. Physical examination showed a pelvic mass anterior to the rectum. Therefore, a pelvic sonogram ( Figure 2) and computed tomography ( Figure 3) were obtained. The pelvic sonogram showed a fluid-filled tubular structure separate from the uterus anteriorly, and from the rectum posteriorly. The computed tomography scan confirmed this slightly left-sided tubular structure. At the time of the CT, no contrast given orally was seen to enter the left-sided tubular structure. The child was taken to the operating room for a better evaluation by means of cystoscopy, vaginoscopy and colonoscopy. Cystoscopy and vaginoscopy were normal without a bladder septation or fistula being identified. A small pit was identified in the rectal wall just above the anus, which was catheterized and contrast injected (Figure 4).The contrast enema at the time of colonoscopy showed a tubular colorectal duplication communicating with the distal Figure 1 Single frontal radiograph of the left hand. The thumb is absent. Figure 2 Longitudinal sonograms of the pelvis. Longitudinal (a) and transverse (b). Behind the uterus (Ut) and anterior to the rectum (R) is a tubular structure (arrow) with an echogenic central structure, related to the mucosa, and a hypoechoic periphery related to the muscular wall. These findings are the characteristic sonographic signatures of the bowel.