The common use of Dacron arterial prostheses produced enormous improvement in vascular surgery, yet some serious complications such as thrombosis or infection persist, which are connected to the utilization of this material. Light and electron microscopic studies may allow a better understanding of such complications. Scanning electron microscopy and semithick sections were used to examine dacron arterial prostheses removed for failure after long-term implantation. Semithick sections showed periprosthetic inflammatory reaction and the neointimal surface never exhibited a regular endothelial layer (at S.E.M.).