OBJECTIVE Composite arterial grafting causes splitting of internal thoracic artery flow to various myocardial regions. The amount of flow supplying each region depends on the severity of coronary stenosis. Competitive flow in the native coronary artery can cause occlusion or severe narrowing of the internal thoracic artery supplying this coronary vessel. METHODS Two hundred three consecutive postoperative coronary angiographies of 163 patients who underwent bilateral internal thoracic artery grafting using the composite-T-graft technique were analyzed. Angiographies were done in symptomatic patients or in patients with positive thallium scan between 2 and 102 months after surgery and were compared with preoperative angiograms. RESULTS In 123 patients, both internal thoracic arteries were patent. The remaining 40 control patients had at least 1 nonfunctioning internal thoracic artery. A lower stenosis rate in the left anterior and circumflex arteries was associated with higher occlusion rate of the left internal thoracic artery (P < .005) and the right internal thoracic artery (P < .005), respectively. In 19 angiograms of 18 patients, graft failure could be related to competitive flow. This included 7 patients with disease of the left main artery and a preoperative stenosis degree ranging between 50% and 80%, 8 patients with moderate stenosis (70% or less) of the circumflex artery, and 3 with moderate stenosis of the left anterior descending artery. Three of the patients with disease of the left main artery, 2 of the patients with competitive flow in the circumflex artery, and all patients in the subgroup with left anterior descending arterial disease underwent percutaneous or surgical reintervention. CONCLUSIONS The composite T-graft technique of bilateral internal thoracic artery grafting should be reserved for patients with severe (70% or more) left anterior descending and circumflex arterial stenosis.