An aneurysm occlusion device is positionable within a cerebral blood vessel covering a neck of an aneurysm on the blood vessel. The device includes a tubular element having a lumen surrounded by an occlusive sidewall including a plurality of gaps. The gaps are sufficiently small to cause at least partial occlusion against flow of blood from the blood vessel through the side wall into the aneurysm, but are expandable in response to a fluid pressure differential between a first area inside the lumen and a second area outside the lumen to allow flow of fluid through the side wall between the blood vessel and a side branch vessel.