1. A self expanding vascular endoprosthesis adapted to bridge across an aneurysm in an artery, the endoprosthesis comprising a substantially imperforate sheet having at least one longitudinal edge and being made of a resiliently flexible biocompatible material, the sheet being rolled upon itself about a longitudinal edge to form an endoprosthesis having two ends, one at either end of the longitudinal edge, the sheet having an inner surface and an outer surface each of which is smooth and free of ribs or other projections that would impede the resilient expansion of the endoprosthesis into contact with an internal wall of the artery both above and below the aneurysm, the material from which the sheet is formed being such that (a) upon being introduced into an artery the endoprosthesis will resiliently expand due to its resilient flexibility to press firmly against an internal wall of the artery both above and below the aneurysm to bridge across the aneurysm and to fluid isolate it from blood flowing in the artery, and (b) the endoprosthesis having sufficient longitudinal stiffness such that there will be a compliance mismatch between the endoprosthesis and a wall of the artery to induce sufficient cellular proliferation in that arterial wall adjacent the ends of an implanted endoprosthesis to cause the endoprosthesis to be adhered to the arterial wall.