In situ saphenous vein bypass grafts originating in the groin were performed in 455 consecutive patients. Primary failure occurred in 92 grafts during follow-up, including 22 (4.8%) with nonocclusive stenosis and 70 (15.4%) with occlusion. The cause for failure could not be determined in seven grafts; 104 contributory causes were identified in the remaining 85 grafts. Among the 104 likely causes, 66 (63%) were intrinsic to the graft itself and contributed to failure of 55 (12.1%) of 455 grafts. These causes included perianastomotic stenosis (48), vein stricture (14), focal vein stenosis (10), valvulotome injury (9), kink (6), retained valve leaflet (4), intimal flap (3), and residual arteriovenous fistula (2). Among these intrinsic causes, 20 were directly related to the in situ technique, contributing to failure of 15 (3.3%) of 455 grafts. Thirty-eight (37%) of the 104 causes were extrinsic to the graft, including compromised inflow (2) or outflow (19), hypercoagulability (9), systemic hypotension (6), and graft sepsis (2). Hypothetically, improvements in technique, patient selection, and perioperative management might have eliminated 46 (44%) of 104 causes of primary graft failure. Delayed graft and anastomotic stenosis and late progression of outflow disease remain resistant to modern therapy.