Polytetrafluoroethylene (PTFE) dialysis grafts in patients with end-stage renal disease (ESRD) are prone to thrombotic failure. The objective of this multicenter, randomized, double-blind, placebo-controlled clinical trial was to determine if warfarin reduces the risk of failure of PTFE dialysis grafts. Patients with ESRD and newly placed PTFE grafts were studied at community and academic dialysis centers in Southwestern Ontario. Patients were allocated to receive warfarin or matching placebo, with the warfarin administered to achieve a target INR of 1.4 to 1.9. Time to graft failure was the main outcome measure. A total of 107 patients (56 allocated to warfarin) were randomized. The time-to-event analysis revealed no significant difference in the likelihood of graft survival between the two groups (odds ratio, 1.76 in favor of placebo; 95% confidence interval, 0.72 to 4.34). Six major bleeds occurred in five patients allocated to warfarin compared with none in the patients who received placebo (P = 0.03). In conclusion, low-dose warfarin was associated with an excess of clinically important major bleeding in patients with ESRD enrolled in this study. Furthermore, low-intensity, monitored-dose warfarin does not appear to prolong PTFE graft survival.