Expanded polytetrafluoroethylene (PTFE) prostheses with fibril lengths of 30 and 60 microns were implanted in the rat infrarenal aorta. Sequential scanning electron and light microscopic studies of the prostheses after implantation demonstrated a different pattern of endothelialization. Prostheses with a fibril length of 60 microns had a continuous multilayered neointima at week 25 postimplantation, whereas prostheses with a 30-microns fibril length had a discontinuous and single layer of endothelium after the same interval. It was concluded, therefore, that a prerequisite for the development of a lining in a vascular prosthesis is for the inner surface of the prosthesis to have adequate pores for effective anchoring of the invading endothelioid cells. Expanded PTFE prostheses with an internodular distance of 60 microns provided sufficient anchoring possibilities for invading endothelioid cells to form a continuous neointima.