Through collaboration of surgeons, pathologists and bioengineers at five centers in Canada and France, this study analyzed the late pathology and structural changes in 73 expanded PTFE arterial prostheses harvested from patients at autopsies and reoperations. The degree of tissue encapsulation increased with the duration of implantation but was reduced by the presence of infection. In several cases, the fibrous tissue penetrated the wall of the prosthesis and partitioned off the thin outer layer, thus disrupting the delicate microporous structure of the wall. The presence of aneurysms was observed in models that had no external reinforcing layer and among grafts that apparently suffered from surgical trauma. Wrinkling of grafts was noted at areas of flexion and was often associated with thickening of the external capsule and reduced luminal diameters. Endothelialization was found within only a few millimeters of the anastomoses. The luminal surfaces were generally not well healed. The PTFE structure was usually readily visible under a thin covering of loosely adhering thrombotic deposits. Bacteria were observed in 46% of the cases, even though only 29% were considered clinically infected. The incidence of lipid or cholesterol deposits was high. Avoiding iatrogenic trauma to the external wall of the prosthesis during implantation is important. Those features where design improvements are required to provide longer term structural integrity and dimensional stability in future models of expanded PTFE prostheses should be identified.