A patient with symptoms of acute myocardial infarction but ST-segment depression rather than elevation constitutes a clinical dilemma for which few guidelines exist. Herein we describe such a patient, in whom serial tomographic imaging with a new radiopharmaceutical agent, technetium-99m sestamibi, was useful in demonstrating a large area of myocardium at risk and subsequent substantial benefit from acute reperfusion therapy. Because this perfusion agent washes out slowly from the myocardium, imaging can be delayed for several hours; thus, acute reperfusion therapy can be performed without delay. Subsequent imaging, however, will reflect myocardial perfusion at the time of administration of the radionuclide. Additional studies with this agent may be valuable in identifying those patients with ST-segment depression who will benefit from acute reperfusion therapy.