The success of aorto-coronary bypass operations, employing an autologous saphenous vein, depends mainly upon graft patency. Fortyone grafts have been examined by light micrsocopy, 35 of which were obtained from 23 autopsy cases and 6 were recovered at reoperation. Thirtythree veins had been in place for less than 20 days (early group) and 8 from 3 to 39 months (late group). In the early group, minimal to moderate mediointimal fibrosis was always detected. Graft occlusion due to recent thrombosis was observed in 13 cases; the cause of thrombosis was severe coronary narrowing distal to the insertion of the graft and atherosclerotic plaque ulceration at the site of the veno-coronary anastomosis. Among the late group, 3 veins were occluded by an old thrombus, and 3 by intimal leiomyocellular proliferation, with Alcian positive ground substance, while 2 exhibited a severe stenosis due to intimal phlebosclerosis. One of these last disclosed a vein atherosclerosis. The multiple pathogenetic factors, which are involved in the proliferative lesions, are reviewed.