Experience with in situ saphenous vein bypasses during 1981 to 1989: determinant factors of long-term patency. From 1981 to 1989, 361 consecutive in situ saphenous vein bypasses were performed. Indications for revascularization were critical limb ischemia (n = 335, 93%), popliteal aneurysm (n = 15, 4%), and claudication (n = 11, 3%). Outflow tract was the popliteal artery in 116 (32%) and tibial artery in 245 (68%) of bypasses. At 6 years primary patency was 63% and secondary patency was 81%. During the performance of the in situ bypass procedure, 86 (24%) venous conduits were modified because of a technical failure (n = 49, 13%) or an inadequate vein segment (n = 37, 10%). Secondary patency at 4 years for bypasses requiring modification was 72% compared to 84% for bypasses not modified (p less than 0.05). Atherosclerotic disease of the inflow artery necessitating endarterectomy, patch angioplasty, or replacement lowered primary patency at 3 years (69%) compared to the inflow artery not requiring reconstruction (46%, p less than 0.02). In the follow-up period, 95 (26%) bypasses were revised because of thrombosis or hemodynamic failure. Bypasses requiring revision had a 4-year secondary patency of 68% compared to 88% for bypasses not revised (p less than 0.02). The first 179 cases (1981 to 1985) were compared to the subsequent 182 cases (1986 to 1989). The secondary patency at 3 years for the latter half (92%) compared to the first half (80%) of the experience was significantly improved (p less than 0.02). The secondary patency for bypasses not requiring revision was significantly improved (p less than 0.02) for the latter half (n = 142, 97%) compared to the first half (n = 124, 83%) of the series. Long-term patency with the in situ saphenous vein bypass is dependent on surgical experience, quality of the venous conduit, and atherosclerotic disease of the inflow artery that necessitates reconstruction. Meticulous surgical technique and compulsive bypass surveillance results in superior long-term patency. 