In contrast to shunt operation the indication for an endoscopic ventriculostomy in patients diagnosed for normal pressure hydrocephalus is not scientifically established. From September 1997 to March 1999 we operated on 36 patients diagnosed for normal pressure hydrocephalus. Diagnosis was established by means of the intrathecal lumbar or ventricular infusion test, the cerebrospinal fluid tap test and MRI-CSF flow studies pre- and post-operatively. In 30 patients (83%) we implanted a ventriculo-peritoneal shunt, and in 6 patients (17%) we performed the endoscopic assisted third ventriculostomy. With our created NPH recovery rate and use of the clinical grading for normal pressure hydrocephalus created by Kiefer and Steudel we compared the operative results of both patient groups. In patients with a pathologically increased resistance to CSF outflow in the lumbar infusion test a shunt implantation is indicated. Patients whose outflow resistance is increased in the ventricular infusion test but with a physiological lumbar infusion test are suspected for a functional aqueduct stenosis and should be treated by means of an endoscopic assisted ventriculostomy.