Since recognition of coronary arterial clloateral circulation in living patients has been made possible by coronary arteriography, controversy has existed about the functional importance of these vessel and their ability to protect the myocardium against ischemia. The coronary arteriograms and left ventriculograms of 279 consecutive patients were reviewed. All had at least 50% diameter reduction of 1 or more major coronary arteries. In 94 patients(111 arteries), at least 1 major branch was totally occluded. Collateral circulation was seen in 85 of 111(76.6%) totally occlued arteries versus 22 of 107(20.6%) with ≥90% but <100% stenosis(P<0.01). No artery with <90% stenosis(254 arteries) recieved angiographically detectable collateral vessels. An analysis was made of the relation between left ventricular segmental wall motion and the quality of collateral circulation in 68 totally occluded arteries among 60 patients with myocardial infarction(Group 1) and in 43 totally occluded arteries among 34 patients without prior myocardial infarction(Group 2). Good collateral vessels went to 62.8% of Group 2, but 38.8% of Group 1(P<0.05). LV contraction was abnormal in all Group 1 patients with good collateral circulation. Of 27 with good collateral circulation in Group 2, LV contraction was normal in 59.3% and abnormal in 40.7%. But there was no statistically significant difference between the effect of good or poor collateral circulation in LV wall motion in each Group.