Background Poor patency rates have limited the success of biological vascular grafts in the coronary artery position.Recently, two bovine internal mammary arterial grafts have been developed for possible use as coronary artery bypass graft (CABG) conduits: (1) Denaflex grafts (Baxter Healthcare Co, 3-mm ID) treated with polyepoxy compounds and with heparin ionically bound to the luminal surface and (2) Bioflow grafts (Bio-Vascular, Inc, 3-mm ID) treated with dialdehyde starch.Methods and Results Thirty dogs underwent CABG with either a Denaflex (n=20) or Bioflow (n=10) graft to the left circumflex coronary artery (LCx).The left main coronary artery (n=12) or proximal LCx (n=18) was then ligated. Six-month patency (Kaplan-Meier) for Denaflex grafts was 44+-13% (+-SEM), compared with 12+-11% for Bioflow grafts, but this difference did not reach statistical significance (P=.56). Among grafts open at 14 days, however, there were no occlusions among six Denaflex grafts versus five occlusions among seven Bioflow grafts. At 6 months, all six surviving Denaflex grafts appeared normal, while the only remaining patent Bioflow graft was angiographically dilated and had diffuse luminal irregularities. At 1 year, three Denaflex grafts angiographically had no dilation, stenosis, or luminal irregularities. Macroscopically, all explanted long-term (6 to 12 months) Denaflex grafts had a smooth, clean luminal surface, whereas the only patent Bioflow graft had multifocal thrombi. Microscopically, all Denaflex grafts had minimal degenerative changes, but the Bioflow graft had transmural linear cracks and medial deterioration.6- month) patency is possible with small-caliber, low-flow biological grafts in the canine coronary position, although both types of grafts are prone to early occlusion. If these early failures are excluded, the Denaflex graft appears to be associated with better long-term patency and an absence of degenerative changes. (Circulation. 1994;90(part 2):II-160-II-166.)