True aneurysm formation in infrainguinal autologous vein grafts is rare. The authors report a case of a patient presenting with extensive aneurysmal degeneration of a femoropopliteal bypass vein graft 13 years after the original operation and 8 years after graft revision and thrombolysis for acute occlusion. Over a 2-year period, the vein graft aneurysm expanded from 2.7 to 4.5 cm maximal diameter. He underwent exclusion and partial resection of the vein graft aneurysm with placement of a new femoropopliteal bypass using reversed saphenous vein from the contralateral leg. Histopathologic examination of the aneurysm revealed intimal fibrosis, medial degeneration, and inflammation. This case and review of the literature suggests that true infrainguinal vein graft aneurysms occur infrequently and massive aneurysm degeneration is even more uncommon. Because of the rarity of true infrainguinal vein graft aneurysms, their etiology remains unclear.