We report the diagnosis of mammary artery graft dysfunction by high-resolution transthoracic Doppler and venous adenosine infusion. The patient was treated by percutaneous balloon angioplasty, with optimal angiographic results. Coronary flow reserve in the distal left anterior descending artery was abnormal before angioplasty, and recovered soon after the procedure. The utility of this new non-invasive technique in the diagnosis of flow-limiting stenoses and follow-up of coronary angioplasty is described.