Atherosclerosis of the internal mammary artery is a rare disease. We describe a case in which a 41-year-old man underwent coronary artery bypass surgery 23 years after repair of aortic coarctation. The diseased mammary artery was used to graft the left anterior descending coronary artery after intraoperative confirmation of good patency. Early graft occlusion occurred a few days after the operation. This case demonstrates that atherosclerosis affects mammary arteries after long-term repair of aortic coarctation. Such conduits should not be used, even in the presence of good blood flow.