A randomized, double-blind clinical trial was designed to assess the effect of aspirin (ASA) alone or in combination with dipyridamole (DIP) on the patency rates of expanded PTFE grafts placed in the infrainguinal position. Forty-nine patients were randomized into three groups who received three times daily either two placebos (17 patients), 325 mg ASA and placebo (16 patients), or 325 mg ASA and 75 mg DIP (16 patients). The patients were seen at 3-month intervals for 1 year, and coded medication bottles were dispensed and returned pills counted to assess patient compliance. Treatment failure was defined as the first graft occlusion. The data were analyzed using the Breslow statistic for progressively censored survival type data. The 1-year cumulative patency rate for the entire series was 59%. The rates for above-knee grafts in the ASA group (100%) and the ASA/DIP group (100%) were significantly higher than the rates for the placebo group (50%) (P = 0.05). The 1-year cumulative patency rates for patients with below-knee grafts were not statistically different among the groups, although the patients who received ASA alone had a higher rate than did the other two groups (65% versus 21% for placebo and 19% for ASA/DIP). There were fewer occlusions in the above-knee grafts as compared to below-knee grafts in all groups, but the differences were statistically significant only in the ASA/DIP group. There were no statistical differences between the two active treatment groups.