In contrast to the shunt operation the indication for an endoscopic ventriculostomy in patients diagnosed for normal pressure hydrocephalus is not scientifically established. Between September 1997 and December 1999 we operated on 48 patients diagnosed for normal pressure hydrocephalus. The 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 postoperatively. In 37 patients (77%) we have implanted a ventriculo-peritoneal shunt, and in 11 patients (23%) 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 groups of patients. 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 endoscopic assisted ventriculostomy.