The performance of knitted Dacron and polytetrafluoroethylene (PTFE) bifurcated grafts are compared in this study of 312 patients at a single institution. Patients of the two graft groups were statistically well-matched in risk factors and degree of distal obstructive disease. Operating time needed to implant either graft was approximately equal. For patients with abdominal aortic aneurysms, mean volume of blood transfused was 2.2 units for Dacron grafts and 0.2 units for PTFE grafts; for patients with aortoiliac occlusive disease, the comparable figures were 1.1 units and 0.1 units, respectively. Four-year cumulative patency for Dacron (90%) and PTFE (97%) grafts were not significantly different (p greater than 0.01). Complications affected 13% of the patients of the Dacron group and 4% of the PTFE group. All six graft infections and all seven graft double-limb thromboses occurred in Dacron grafts. Anastomotic aneurysms, amputations, and late graft revisions occurred with greater frequency in patients with Dacron grafts.