Rupture of arteriovenous fistula after in situ saphenous vein bypass. Residual saphenous vein tributaries function as arteriovenous fistulas after in-situ lower extremity bypass. Whether or not all of these tributaries need to be ligated at the time of bypass is controversial since many will close spontaneously. Other consequences of retained fistulas range from local skin problems to diminished graft flow and ultimate graft thrombosis. A complication not previously reported is rupture of a retained arteriovenous fistula. This unusual and potentially catastrophic complication should be discoverable at an earlier stage by periodic graft surveillance by means of duplex sonography. The routine policy of selective ligation of saphenous vein tributaries needs to be reexamined. 