The first case of congenital occlusion of the aqueduct of Sylvius was reported by Hilton in 1847. Pancoast, Pendergrass, and Schaeffer have stated in their classic work, “The Head and Neck in Roentgen Diagnosis,” that of all of the lesions revealed by ventriculography those obstructing the aqueduct are the most difficult to diagnose. Twining emphasized the danger of being misled by inadequate visualization of the aqueduct and fourth ventricle and discussed the hydrodynamics involved in filling these structures with air. Complete replacement of cerebrospinal fluid with air is seldom possible by ventricular puncture in the presence of a blocked ventricular system. The difficulties encountered, therefore, are largely technical and involve positional manipulation of the available air to establish the point of occlusion of the system. However, there are also the possibilities of an incomplete block of the aqueduct and unsatisfactory visualization of an air-filled aqueduct due to superimposed structures that ob...