A 48 year old woman with a massive retrorectal tumor extensively invading the sacrum is presented herein. A submucosal tumor, apparently of rectal origin, was found on the posterior rectal wall infiltrating posteriorly into the sacrum. Abdominosacral resection of the rectum with partial sacrectomy was thus performed and postoperative examination revealed the tumor to be carcinoid which has hitherto been associated with moderately benign behavior. Although remote metastasis was absent, extensive lymph node metastasis was observed, however, 2 years have passed since the operation without any evidence of recurrence.