Cardiac ischemia after coronary artery bypass grafting is often caused by graft occlusion. Short- and long-term graft patency is related to the quality of the surgical technique during harvesting and anastomosis. Transit time flow measurement is a recognized technique for the quality control of grafts but may not rule out structural abnormalities in the conduits, which can cause graft occlusion. This article reports on two cases of suspected intra-arterial dissection of the left internal mammary artery despite satisfactory flow measurements. Routine ultrasound scanning of arterial conduits is helpful in distinguishing dissection and hematoma in the graft conduits.