The Coronary Artery Surgery Study (CASS) includes a randomized trial of coronary artery bypass surgery and medical therapy in the management of patients with mild or moderate stable angina pectoris or free of angina but with a documented history of myocardial infarction. While 780 patients at 11 participating institutions entered the randomized trial, 1,315 patients at the same institutions met randomization criteria but declined participation in the randomized study; they constitute the "randomizable" patients. Half the randomized patients were assigned to surgery and half to the medical group. Of the 1,315 randomizable patients, 43% started with surgical therapy and 57% constitute the medical group. Follow-up periods average 64 months (range 46 to 92). The only entry characteristic in which the randomized and randomizable medical groups differ importantly is the extent of coronary artery disease, which is less extensive in the latter. The two surgical groups also differ in this respect, but with more extensive disease in the randomizable group. At 5 year follow-up, 24% of the medically-assigned randomized patients and 22% of the medically-started randomizable patients have had coronary bypass surgery. Survival in the medically-randomized and randomizable patient groups is similar in the aggregate (both 92% at 5 years) and also in all subgroups based on clinical classification, the number of diseased vessels, the presence of proximal left anterior descending coronary artery disease and ejection fraction. Survival for the surgically-assigned randomized patients and the surgically-started randomizable patients is also similar in the aggregate (95 and 94%, respectively) and in all subgroups. It is concluded that the randomized patients in CASS are not a special or atypical subset of those eligible for randomization. The data from the randomizable patients thus support and extend the inference of the generally very good survival of both the medically- and surgically-assigned patients of the randomized trial.