In two patients, venous loops fashioned into arteriovenous anastomoses for long-term hemodialysis developed gross irregularity and narrowing of the lumen. Histological and ultrastructural examination of the affected vein in one patient revealed atherosclerosis similar to that that develops in the anastomosed veins of experimental arteriovenous fistulas in sheep. The second patient, with angiographically similar changes in the venous loop, developed on aneurysm followed by thrombotic occlusion of the arteriovenous fistula. The development of venous atherosclerosis in these patients was analogous to that in the experimental animal, and was attributed to local hemodynamic stresses in the vein associated with the shunt.