We have reported a rare case of spontaneous 3rd ventriculostomy with spontaneous arrest of obstructive hydrocephalus. A 41 year old man, who had had an intermittent headache for about a year, was admitted to the department of neurosurgery Kitasato University with chief complaints of sudden onset of severe headache, vomiting and disturbance of consciousness. At the time of admission, 30 minutes after the onset of symptoms, the positive neurological findings were delirious state of consciousness, miotic pupils with sluggish reaction to light, mild hemiparesis on the left site and slight nucnal rigidity. He lapsed into coma after two hours, however he gradually relieved from these symptoms since the forth hospital day. Cerebrospinal fluid was bloody. Radiograms of the skull revealed decalcification of posterior clinoid process and postero-inferior displacement of pineal calcification. Brain scanning and vertebral angiography demonstrated tumor stain in the posterior portion of the 3rd ventricle. Dimer-X ventriculography revealed the obstruction of posterior portion of the 3rd ventricle and the leakage of Dimer-X through the floor of the 3rd ventricle into the intrasellar subarchnoid space. The patient died after about one year from the onset of symptoms. Any signs of increased intracranial pressure had not been noticed since the forth hospital day; At autopsy we confirmed the posterior portion of the 3rd ventricle was obstructed by tumor. In the floor of the 3rd ventricle there was a round opening which was patient and measured about 3 mm in diameter. Microscopic examination of the tumor showed an oligodendroglioma. Neoplastic cells partially infiltrated into the surface facing to the 3rd ventricle and slight gliosis was observed around the site of rupture. The surface along the subarachnoid space was lined with pia-aracnoid membrane except at the site of rupture. In the past literatures only 6 cases of spontaneous 3rd ventriculostomy have been reported. Three cases were observed spontaneous arrest of obstructive hydrocephalus. Our case is the first reported case of spontaneous 3rd ventriculostomy through the floor of the 3rd ventriculostomy through the floor of the 3rd ventricle. We suggested the pathogenesis of spontaneous 3rd ventriculostomy is a result of destruction at normally weak points of 3rd ventricle (ex. anterior, posterior wall and floor of 3rd ventricle), which has the reultant internal hydrocephalus caused by recurrent obstruction of C.S.F. pathway or long-standing obstructive hydrocephalus.