Preoperative coronary arteriograms were correlated, in a group of 50 patients, with left internal mammary angiograms obtained from 11 to 32 months, with a mean of 17 months, after mammary artery implantation. In all patients in whom the internal mammary artery was patent and considered functional with good angiographic opacification of the anterior descending coronary artery, the preoperative coronary angiogram showed total or subtotal obstruction of the latter vessel, with indirect evidence of decreased flow and pressure distal to the obstruction. This evidence was provided by the presence of a collateral circulation or, in a few cases of subtotal obstruction, delayed opacification of the vessel distal to the obstruction.In patients in whom the internal mammary artery was patent but showed no anastomotic connection with the anterior descending coronary artery or only opacification of small coronary branches, the degree of coronary obstruction was, in most cases, less than 90% of the lumen of the coronary artery in the absence of any collateral circulation or delayed opacification of the vessel distal to the obstruction.Occlusion of the internal mammary artery was seen as often in the presence of total or subtotal obstructions as with lesser degrees of anterior descending coronary artery obstruction, and is believed unrelated to the degree of pre-existing coronary artery disease.Successful internal mammary artery implantation can be related to specific coronary angiographic patterns recognizable before operation; these may serve as reliable criteria for the selection of patients.