The performance of completely biological, decellularized engineered allografts in a sheep model was evaluated to establish clinical potential of these unique arterial allografts. The 4-mm-diameter, 2-3-cm-long grafts were fabricated from fibrin gel remodeled into an aligned tissue tube in vitro by ovine dermal fibroblasts. Decellularization and subsequent storage had little effect on graft properties, with burst pressure exceeding 4000?mmHg and the same compliance as the ovine femoral artery. Grafts were implanted interpositionally in the femoral artery of six sheep (n=9), with contralateral sham controls (n=3). At 8 weeks (n=5) and 24 weeks (n=4), all grafts were patent and showed no evidence of dilatation or mineralization. Mid-graft lumen diameter was unchanged. Extensive recellularization occurred, with most cells expressing αSMA. Endothelialization was complete by 24 weeks with elastin deposition evident. These completely biological grafts possessed circumferential alignment/mechanical anisotropy characteristic of native arteries and were cultured only 5 weeks prior to decellularization and storage as "off-the-shelf" grafts.