Transthoracic Doppler echocardiography (TDE) is a non-invasive and easy reproducible method to assess the left internal mammary artery (LIMA) graft patency after coronary artery bypass graft surgery (CABG). LIMA graft dysfunction is rare, its rate being 10% at 10 to 15 years after revascularization. The most common cause of graft dysfunction is the competitive flow with the native coronary artery, when the stenosis of the bypassed vessels is not severe. We present two cases of LIMA graft dysfunction diagnosed by TDE and confirmed by angiography, with two particular pulsed-wave color Doppler flow signals.