PURPOSE: Migration of endoprostheses remains a concern in endovascular aneurysm treatment. Biologic fixation is supposed to enhance anchorage, but the diseased atherosclerotic aorta in humans has demonstrated a limited capacity to incorporate an endoprosthesis by cellular proliferation. The biologic response of two different types of endoprostheses was evaluated in the porcine aorta. MATERIALS AND METHODS: Two types of endoprostheses--four polyurethane-covered (PUC) stents with a macroporous polyurethane covering and four polyester-covered (PEC) stents with a woven polyester covering--were implanted in eight infrarenal porcine aortas for 6 weeks. Electron microscopy and qualitative and quantitative microscopy were performed on serial cross sections. RESULTS: The PUC stents demonstrated an increase in diameter (from 8 mm +/- 1 to 10 mm +/- 1, 12.5%; P = .009), whereas the PEC stents persisted in their original dimensions (8 mm +/- 1, 0%). PUC and PEC stents were covered by continuous thrombus-free neointima (269 microm +/- 51 vs 575 microm +/- 113, respectively; P < .01). The PUC stents demonstrated firm attachment to the aorta as a consequence of a granulation tissue with ingrowth into the pores of the polyurethane covering. The PEC stents remained in loose contact with the aorta in the absence of tissue ingrowth. CONCLUSIONS: Enhanced biologic fixation was achieved by extensive granulation tissue invading the pores of PUC endoprostheses. This finding can modify the design of future devices.