Background. In coronary arteries with moderate stenosis, competitive flow may lead to internal thoracic artery (ITA) graft occlusion. The goals of this study were to determine if competitive flow reduces ITA patency, and if there is a degree of coronary stenosis below which ITAs should not be used. Methods. From 1972 to 1999, 50,278 patients underwent primary coronary artery bypass grafting (CABG). Of these, 2,002 had at least one ITA graft and postoperative angiography before coronary reintervention; 2,999 angiograms of 2,121 ITAs were made. Time-related ITA occlusion was modeled using longitudinal analysis to identify its risk factors while accounting for lack of independence introduced by repeated angiography and multiple ITA anastomoses per patient. Proximal coronary stenosis (maximum preoperative stenosis between ITA anastomosis and aorta) was the surrogate for competitive flow. Results. Unadjusted ITA patency was 93%, 89%, 90%, and 92% at 1, 5, 10, and 15 years after CABG. Risk factors associated with ITA occlusion were lesser degree of proximal coronary stenosis (p < 0.0001); longer time from CABG in grafts to non-left anterior descending coronary arteries (p < 0.0001); female sex (p = 0.0003); later date of CABG (p = 0.01); right ITA (p < 0.0001); and smoking (p < 0.0001). In all arteries, as preoperative proximal coronary stenosis decreased, ITA patency declined; however, at no degree of stenosis was there a sharp decline. Conclusions. Internal thoracic artery patency decreases as coronary competitive flow increases. However, the nature of this relationship indicates ITAs should not be abandoned at moderate grades of stenosis.