Effects of duration of acetylsalicylic acid (ASA) administration on the patency and development of distal anastomotic intimal hyperplasia (DAIH) of endothelial cell (EC) seeded and unseeded prosthetic aortoiliac grafts were studied in a canine model. ASA, 5 gr po qd, was administered to dogs 1 day prior to placement of bilateral, 12 to 17 cm long, 5 mm inside diameter expanded polytetrafluoroethylene (ePTFE) aortoiliac grafts and continued for 2 wk (Group 1, n = 12 dogs) or 16 wk (Group 2, n = 12 dogs). Six dogs in each group received autologous EC seeded grafts, while the others received unseeded grafts. Prosthesis patency was assessed weekly. At the conclusion of the study, DAIH was measured on serial sections using a computer-linked digitizer. The 16 wk patency for Group 1 grafts was 67%, while that for Group 2 grafts was 88% (p less than 0.09). Luminal narrowing due to DAIH was not significantly different between Groups 1 and 2 (7.7 +/- 8.3% [means +/- SD] and 9.0 +/- 7.8% respectively). EC seeding improved the 16 wk combined patency from 62 to 92% (p less than 0.02). A more complete luminal endothelial cell lining was correlated with reduced DAIH (r = -0.4, p less than 0.05). Chronic ASA administration prevented graft thrombosis between 2 and 4 wk postimplantation in this study but was not associated with decreased DAIH.