A promising new operation-coronary bypass-has been developed for the treatment of coronary heart disease. However, because of the nature of the underlying disease and the minute size of the vessels to be anastomosed, its long-range prospects are in doubt. At this point in time there is no evidence that the operation either prevents infarction or prolongs life, and great caution is urged in applying it, reserving it for those who have life-threatening coronary artery disease, are clearly recalcitrant to optimum medical management, and have apparently normal coronary vessels distal to the point of stenosis.There is an urgent need for a better definition of the indications for operation in patients with coronary artery disease based on a study of the natural history of this coindition. Prophylactic operation for patients with a compromised coronary circulation with or without angina must await long-term results of therapeutic operation.