Modification procedures performed during in situ bypass grafting to correct an injured or inadequate saphenous vein segment result in a significant increase in the incidence of vein graft complications in the follow-up period. Modification procedures were performed in 96 in situ saphenous vein bypasses and consisted of primary closure (n = 28), vein patch angioplasty (n = 31), or resection and/or replacement (n = 37). At 4 years primary patency was 54%, secondary patency was 73%, and limb salvage was 89%. The incidence of subsequent vein graft stenosis and revision or graft failure was similar for grafts requiring vein patch angioplasty (7 of 31, 23%), primary repair (9 of 28, 32%), and resection and/or replacement (16 of 37, 43%) (p not equal to ns). Only 4 bypass revisions were performed for stenosis at the site of the original modification procedure. The type of vein graft repair did not significantly affect the primary patency at 18 months (primary closure, 65%, vein patch angioplasty, 66%, and resection and/or replacement, 58%) or the secondary patency at 30 months (primary closure, 80%, vein patch angioplasty, 90%, and resection and/or replacement, 77%). Modified autogenous conduits maintain patency and limb salvage but are prone to develop graft complications in the follow-up period.