A detailed pathological study was made in 10 patients dying up to 13 months after aortocoronary saphenous vein bypass grafting for coronary atherosclerosis. The coronary arteries and vein grafts were investigated by injection with a radio-opaque mass, radiography, dissection, and histology. The report is to some extent historical since the patients died during a period when the operation was first being introduced into two cardiothoracic hospitals. About 80 operations were performed during the time the 10 deaths occurred, a mortality of 12-5 per cent (including cases followed up to 13 months after operation). Seven of the patients were operated on for intractable angina and 3 with a view to aneurysmectomy. All the patients selected for operation were severely disabled despite medical treatment. The main cause of death was extremely severe coronary artery disease and its effects on the left ventricle; in one case, over two-thirds of the left ventricle had been destroyed by infarction before operation. Other causes or contributing causes of death were pulmonary embolism, myocardial infarction complicating angiography (ostial stenosis), and cerebral damage. Ten of the 14 vein grafts (71%) were patent at necropsy. A free flow of injection medium usually occurred between patent grafts and coronary arteries. Thrombosis of a graft was thought to have contributed to death in 3 patients, but not in a fourth who died of pulmonary embolism. Since thrombosis of grafts was usually secondary to poor run-off blood into severely atheromatous coronary arteries, this was also an indirect effect of the advanced coronary arterial disease. In one case, thrombosis followed severe chronic intimal thickening of a graft in place for 13 months. The study of these deaths emphasizes that in some patients the pathological changes in the coronary arteries and left ventricle are too severe for them to benefit from surgery. Vein grafts cannot be expected to distribute blood effectively through grossly narrowed coronary arteries. In addition, when a large part of the left ventricle is infarcted or scarred, it is almost certain that improving the blood supply by grafting will not result in significant regeneration of cardiac muscle. Since the time when this study was made, there have been few deaths among the many vein graft operations subsequently carried out in the hospitals involved. The two most important factors thought responsible for the improvement are the selection of cases more suitable for surgery by continued improvement of diagnostic techniques, and also the employment of more radical surgical procedures in the form of coronary endarterectomy and the insertion of more grafts per patient.