An occlusion device (30) includes at least one self-expanding frame (42, 44) and graft material (46). Occlusion occurs by constricting the graft material (46) at a substantially central point or at an end of the device. The constriction is sized such that a guide wire (34) is able to pass therethrough. A valve mechanism (50) includes a screen (52) made from a resilient material and which includes a closable opening (54). The valve mechanism (50) allows a guide wire to pass through the occlusion device (30) whilst an effective physical barrier is maintained after removal of the guide wire (34).
