Rectal duplication cyst is a rare congenital lesion which is known to be associated with other congenital defects, especially genitourinary and vertebral anomalies. Infections with fistulization, bleeding, and malignant degeneration are the major complications of developmental cysts. The case of an 83-year-old woman referred for acute constipation associated with abdominal distension is reported. CT and MRI showed a large cystic mass of the pelvis with extrinsic compression of the rectum. Surgical excision would have been the treatment of choice. In this case, the patient was unfortunately not eligible for surgery due to her poor general condition but responded well to conservative treatment