The purpose of the present study was to examine the effects of cilostazol on endothelial cell denudation and proliferation in vein grafts used as arterial substitutes. Unilateral aortoiliac bypass was performed using the lateral jugular vein in 20 mongrel dogs. The animals were divided into two groups according to whether or not cilostazol was given. The grafts were removed at intervals of 1 day and 50 days, and the luminal surface was assessed for endothelial cell coverage (%). The denudation of endothelial cells was less extensive in the cilostazol group than in the control group on postoperative day 1. There was significantly more proliferation of endothelial cells in the control group over the course of time than in the cilostazol group. In conclusion, cilostazol significantly prevented early endothelial cell denudation, although it did not appear to stimulate successive endothelial cell proliferation. Therefore, cilostazol may help preserve an intact intima, which would potentially result in the inhibition of intimal hyperplasia.