The long-term benefit of coronary bypass surgery in terms of longevity and prevention of major ischemic events in patients who have mild angina is not well defined. The randomized Coronary Artery Surgery Study (CASS) was designed to evaluate this issue; it consists of 780 patients who were considered operable and who had mild stable angina pectoris or who were free of angina after infarction. As a result of the randomization process there were no significant differences in base-line variables between patients randomly assigned to medical and to surgical therapy. The likelihood of death in the five-year period after randomization was only 8 per cent in the medical cohort, as compared with 5 per cent in the surgical cohort (not significant). The likelihood of nonfatal Q-wave myocardial infarction was 11 and 14 per cent, respectively (not significant). The five-year probability of remaining alive and free of infarction was 82 per cent in the patients assigned to medical therapy and 83 per cent in the patients assigned to surgery (not significant). There were no statistically significant differences in the survival rate or in the myocardial-infarction rate between subgroups of patients randomly assigned to medical and to surgical therapy when they were analyzed according to initial group assignment, number of diseased vessels, or ejection fraction. Therefore, as compared with medical therapy, coronary bypass surgery appears neither to prolong life nor to prevent myocardial infarction in patients who have mild angina or who are asymptomatic after infarction in the five-year period after coronary angiography.