The authors report their experiences of endoscopic third ventriculostomy in the acute obstructive hydrocephalus to emphasize unreported significant findings, such as collapse of the ventricular system subsequent to introduction of endoscope and thick, tough, and opaque floor of the collapsed third ventricle. Operators are required to manipulate neuroendoscope freely to overcome these difficulties, based on der:ailed knowledge of intraventricular anatomy and preoperative evaluation of magnetic resonance (MR) images to locate vessels in the basal ci!;tern.