An anti-siphoning valve for use in hydrocephalic shunt systems is described. The addition of this valve to the system effectively reduces the hazard of negative intraventricular pressure when the patient is sitting or standing. The formation of post-shunt subdural hematomas was prevented by temporary postoperative occlusion of the shunt using a percutaneously reversible occlusion valve, which is also described. KEY WORDShydrocephalie shunts hypotension subdural hematoma 9 reversible occlusion valve siphon effect ventricular anti-siphon valve 1.