Background: The size of the autologous vein graft is an important determinant of prognosis in patients with chronic limb-threatening ischemia (CLTI) undergoing bypass surgery. However, it is unclear whether reconstruction using a small vein graft would directly affect prognosis, or a subgroup having small veins would be subject to a poor prognosis. If the latter is true, having small veins would work as a marker for a poor prognosis even in patients with CLTI undergoing endovascular therapy (EVT). Therefore, we investigated the association between the size of saphenous veins and prognosis in patients with CLTI undergoing EVT.Methods: This study included 209 consecutive patients with CLTI who primarily underwent EVT between January 2017 and December 2018. The diameter of the saphenous vein was measured at three locations (saphenous-femoral junction, knee, and ankle joint) per limb before EVT by ultrasonography. A small saphenous vein was defined as the mean diameter <3.0 mm.Results: All-cause mortality rate was significantly higher in patients with a small saphenous vein than in those with non-small one (46.3% versus 10.7% at 2 years, p<0.001). The multivariate Cox regression analysis revealed that a small saphenous vein was independently associated with an increased risk of all-cause mortality.Conclusions: Having a small saphenous vein was an independent marker for all-cause mortality in patients with CLTI undergoing EVT.