We performed a study on six fresh cadavers to measure the subarachnoid pressure of the optic nerve and to determine its relationship to intracranial pressure. A lateral orbitotomy via a coronal flap was used to expose the optic nerve. Various intracranial pressures were achieved by saline infusion through a ventriculostomy. The subarachnoid pressure of the optic nerve was measured at 5 and at 25 mm posterior to the globe under conditions of the created intracranial pressure. The effectiveness of optic nerve sheath decompression performed by two standardized surgical techniques was determined experimentally and by theoretic calculations; the role of the bulbous portion of the optic nerve was assessed experimentally. Measurements of the subarachnoid pressure of the optic nerve showed interindividual variation and a linear relationship with the intracranial pressure. Surgical decompression through a 3 x 5-mm window was more effective than were three linear 5-mm incisions. The bulbous portion of the optic nerve was found to have an important role in cerebrospinal fluid circulation.