The in situ saphenous vein (ISSV) graft has shown promise in distal bypass. Although improved patency has been attributed to preservation of vasa vasorum, there is no direct evidence to support this hypothesis. Femorodistal bypass was done in 33 patients using ISSV grafts (21) or nonreversed saphenous vein (NRSV) grafts (12) during an 18-month period. The NRSV were completely removed from the vein bed but were otherwise prepared in an identical fashion to the ISSV. Immediate complications including incomplete valvulotomy (one), intimal laceration (one), persistent AV communication (two), and extrinsic graft compression (one) were identified and corrected. Two grafts of 2.5 mm diameter occluded acutely. There were no deaths. Of 30 patients discharged with a patent graft, there was one late occlusion (ISSV) at 10 months. No difference in patency between ISSV and NRSV grafts was noted during follow-up extending to 24 months. Overall limb salvage was 94%. In a canine model, 60 vein segments were interposed in the carotid artery using in situ, reversed, and nonreversed techniques. Ultrastructural studies 1, 2, 3, and 6 months after implantation reveal no differences in in situ and nonreversed grafts. New vasa vasorum were identified in NRSV within 1 month. Both ISSV and NRSV grafts demonstrate excellent patency and maintenance of smooth muscle cell architecture. Factors including reduced size disparity at the proximal and distal anastomoses, physiologic distension under arterial pressure, careful handling, and meticulous technique appear to be more important than the theoretic advantages of preserving vasa vasorum.