We have developed a composite vascular prosthesis with the inner blood interfacing layer made of a porous elastomeric membrane of spun Corethane fibers (10-15 microns in diameter, pore size 30-60 microns) impregnated with a gelatin-heparin complex and an outer soft tissue interfacing warp knitted Dacron sheath. Forty-eight Corethane/Dacron composite (CDC) vascular grafts, 4 mm in diameter and 5 cm in length, and 48 expanded polytetrafluoroethylene (ePTFE) vascular grafts of similar size were implanted bilaterally as end to end interposition grafts on opposite sides in the carotid and femoral arteries in dogs on low dose (162 mg/day) aspirin. Patency at 3 months was 37/48 (77%) for the ePTFE grafts and 44/48 (91.7%) for the CDC grafts. Fisher's exact test (two-tailed comparison) indicates the probability of superior CDC graft patency to be 95.5%. In the seven pairs of grafts in which one graft was occluded and the contralateral graft of another type was patent, the CDC graft was patent and the ePTFE graft was occluded in every instance. Explant histology and scanning electron microscopy showed greater collagenization, less residual platelet-fibrin and red blood cell deposits, and more frequent mid-graft endothelialization in the CDC grafts than in the ePTFE grafts.