Vein-to-artery grafts develop areas of endothelial loss with fibrin and leukocytes which lead to early thrombosis and may lead to subsequent atherosclerosis of the graft. En face monolayers were prepared which removed >90% of vascular intima. Unevenly distributed leukocytes and endothelial cells were counted using a standardized sampling of calibrated oil immersion fields of 0.01 mm2. Nongrafted veins had 14 ± 1 evenly arranged endothelial cells per field without gaps or leukocytes, while 10-min grafts had 13 ± 2 with rare leukocytes. Four-hour grafts from normal dogs had 9 ± 2 endothelial cells with gaps and 97 ± 37 neutrophils and 44 ± 25 monocytes. Leukopenic dogs (vinblastine-treated) had normal numbers of endothelial cells (14 ± 1) with scanty leukocytes. We conclude that leukocytes cause endothelial loss in vein-to-artery grafts that can be prevented by intense leukopenia. This may lead to practical approaches to protecting such grafts in humans.