Accidental subdural extra-arachnoid block is a rare but life-threatening complication of epidural anesthesia. The subdural area is a potential space between the dura and the subarachnoid membranes. It exists in the spinal meninges just as it does in the cranial meninges. In the past 19 years a number of clinical reports have described the unintentional catheterization to this potential space and delayed subdural migration of the epidural catheter. We had three cases of accidental subdural blocks recently. Tbis is a report concerning a case confirmed by subduragram. A healthy 44 years old woman underwent a radical hysterectomy under continuous epidural anesthesia combined with general anesthesia. Epidural catheterization was carried out smoothly and the induction of general anesthesia was uneventful. However, an unexpected hypotension was continuously noticed after the epidural injection of 2% lidocaine 20 ml. Thereafter, general anesthetic was turned off and the respiration was controlled using 100% oxygen. The patient remained unconscious with severe miosis for one and a half hours. 5 ml of a water soluble Niopam 300 was injected through the catheter postoperatively. It was later observed on the subduragram that the catheter was inadvertently misplaced in the subdural space. The anatomy and physiological changes related to subdural block are described hereafter.