In the past, surgeons thought that severe presacral hemorrhage during proctectomy was caused by damage of the presacral venous plexus. By studying the anatomy and clinical data, we found that injury of the sacral basivertebral vein also caused this serious complication. Presacral hemorrhage is seen as massive bleeding from the distal pelvic surface of sacrum or from one to several large-caliber foramina of sacral basivertebral veins in that area. This type of presacral hemorrhage is more dangerous than that from simple injury of presacral venous plexus and sometimes it is fatal. We describe the anatomic features of the vertebral venous system and its close relationship with severe presacral hemorrhage. We also propose some new concepts about cause, hemostatic measures, and principles of prevention.