Abstract: An adjustable-resistance anti-siphon device for a cerebrospinal fluid shunt is provided. The device has a substantially rigid housing having an internal cavity which contains a first chamber and a second chamber, the first chamber having at least one flexible wall, an inlet port and an outlet port, the ports passing through apertures in the housing; the second chamber being gas-filled and being in pressure communication with at least a portion of the at least one flexible wall of the first chamber; and gas pressure control means connected to said gas-filled chamber whereby the pressure in said gas-filled chamber may be adjusted.