Unligated side branches of the left internal mammary artery (LIMA) have been indicated as a cause of coronary steal resulting in postoperative angina. Although a number of studies have reported successful embolization of the side branches to relieve angina, this phenomenon is still controversial and it has been either emphasized or rejected in studies that attempted to obtain hemodynamic evidence of the steal using angiographic and intravascular Doppler techniques. In this case study, we tried to reproduce physiological decrease in the muscular and coronary beds as it could occur during activity. Our results, using an intracoronary Doppler wire with adenosine combined with forced ventilation, showed that a trial occlusion of the LIMA side branch with a balloon technique can demonstrate whether the flow through the LIMA would increase after embolization of the side branch.