Inappropriate use of the internal mammary artery for complex coronary artery bypass operations may have disastrous yet avoidable consequences. In 712 consecutive coronary artery bypass operations performed between January 1985 and September 1986, five patients had an intraoperative course suggesting internal mammary artery hypoperfusion. In three, coronary artery bypass was performed as a reoperative procedure. The following similarities were noted in all five patients: a technically satisfactory internal mammary artery anastomosis, a left anterior descending coronary artery having critical proximal stenosis (greater than or equal to 90%), a distal artery at least 2.0 mm diameter free of disease, satisfactory left ventricular function, a catastrophic clinical outcome (death in three, cardiac transplantation in one, 3-month hospitalization in one), and all patients could be weaned from cardiopulmonary bypass only after insertion of a saphenous vein graft distal to the internal mammary artery graft. Bilateral internal mammary grafts were used in four of five patients. In both patients without prior coronary artery bypass, there was a discrepancy in internal mammary artery flow and myocardial demand: cardiac hypertrophy, large (2.5 mm) distal left anterior descending supplied by a small (less than 2.0 mm) internal mammary artery, and sequentially grafted arteries. All three patients having reoperative coronary artery bypass had normal anterior wall motion, and in two of the three patients, the left anterior descending-saphenous vein graft was arteriographically normal and the sole source of blood to the anterior, septum, and inferior myocardium. In two patients the normal left anterior descending-saphenous vein graft supply a large left anterior descending (greater than 2.0 mm) was replaced with a small left internal mammary artery. Internal mammary artery hypoperfusion typically occurred 30 to 40 minutes after discontinuation of cardiopulmonary bypass but was also seen in the intensive care unit. The condition may be confused with internal mammary artery or coronary artery spasm. It may be avoided by careful prebypass planning of the operation, intraoperative assessment of internal mammary artery flow and size with prudent use of sequential internal mammary artery grafting, avoiding use of the terminal left anterior descending (unless large), retaining nonobstructed saphenous vein grafts when the internal mammary artery has marginal flow or size, and placement of a saphenous vein graft distal to the left internal mammary artery when size or flow in the latter is small.