Unligated side branches of the left internal mammary artery (LIMA) have been described in the literature as a cause of coronary steal resulting in angina. Despite a number of studies reporting successful side branch embolization to relieve symptoms, this phenomenon remains controversial. Hemodynamic evidence of coronary steal using angiographic and intravascular Doppler techniques has been supported by some and rejected by others. In this case study using an intracoronary Doppler wire with adenosine, we demonstrate that a trial occlusion of the LIMA thoracic side branch with selective balloon inflation can confirm physiologic significant steal and whether coil embolization of the side branch is indicated.