Symptomatic occult hydrocephalus with normal cerebrospinal fluid pressure has recently received considerable attention in the neurological literature. Clinically this syndrome may be manifested by progressive dementia, psychomotor retardation, memory impairment, spasticity, and gait disturbance (1, 9–11). Messert and Baker (11) suggested that its cause may be either “idiopathic or nonobstructive” white matter degeneration or obstruction of cerebrospinal fluid pathways by a variety of lesions. Treatment usually consists of shunting cerebrospinal fluid from the ventricular system to the heart or the pleural cavity. Dramatic improvement of symptoms following shunting procedures has been reported, thus adding impetus to the need for accurate diagnosis (1, 10, 11, 13). The diagnosis of occult hydrocephalus is best made by pneumoencephalography. This study deals with the diagnosis of cerebrospinal fluid obstruction at the level of the tentorial incisura (incisural block). The obstruction of the cerebrospinal fluid pathway at the level of incisural notch of the tentorium may be a sequel of any inflammatory process, such as meningitis, encephalitis, or spontaneous or post-traumatic subarachnoid hemorrhage (13). The inflammatory process produces local or diffuse meningeal adhesions (8, 11). The tentorial incisura is the most vulnerable area for a block because of its dependent basilar position and relatively narrow cerebrospinal fluid spaces. The characteristic findings in pneumoencephalography in incisural block are generalized ventricular dilatation, failure of air to pass above the incisural level, and nonfilling of the supratentorial subarachnoid spaces and the convexity of cerebral hemispheres (2, 14). The accuracy of pneumoencephalography in the diagnosis of incisural block depends, however, not only on the skill and diligence of the operator, but also on the specific anatomic peculiarities of the area under investigation. At times even exhaustive attempts to fill the supratentorial subarachnoid spaces and the convexity of cerebral hemispheres with a small amount of air will be unsuccessful and may not be indicative of pathology (6). Di Chiro (4) introduced radioiodinated serum albumin (RISA) cisternography to study cases of cerebrospinal fluid rhinorrhea. This method is easily carried out, and serial observations should demonstrate the dynamics of flow of the cerebrospinal fluid which cannot be studied by air. Bell (2) observed only one patient in whom mild hyperpyrexia developed after the intra-ventricular injection of RISA. Detmer and Blacker (3) reported a case of aseptic meningitis secondary to intrathecal injection of RISA; the symptoms and signs disappeared four days after the onset of meningitis. Di Chiro (4, 7) reported that none of his patients had untoward effects referable to the placement of RISA within the cerebrospinal fluid.