A seventy two year old man presented to the Emergency Department with clinical features of colonic obstruction. Subsequent radiological investigations confirmed this impression and revealed the aetiology to be compression of the sigmoid colon against the sacrum by a massively distended urinary bladder. Chronic urinary retention due to benign prostatic hypertrophy is an extremely unusual cause of large bowel obstruction. Little in this patient's clinical findings suggested this aetiology. We reviewed the literature in this area and highlight the benefits of CT scanning over contrast studies.