The presence of a presacral mass on CT in a patient who has undergone abdominal perineal resection for carcinoma of the rectum may represent a problem in clinical management. We are unaware of any previous report describing normal (but displaced) structures as a cause for this finding on CT. Knowledge of this occurrence could prevent unnecessary biopsy. This report describes three cases in which normal postoperative anatomy simulated a presacral mass on CT after abdominal perineal resection. The nature of this mass was elucidated with MR imaging, thus obviating invasive procedures.