Forty-six patients admitted with acute coronary insufficiency are reviewed. All were investigated by coronary angiography; 4 had normal coronary arteries and are included in this study; the remainder had a distribution of coronary artery disease similar to other angina patients. The clinical and angiographic findings, management, and subsequent course of the other 42 patients are presented. Fourteen patients (33%) in whom rest pain persisted after 48 hours underwent emergency coronary angiography, with 3 deaths; of the surviving 11 who had acute saphenous vein bypass grafting, 2 died at operation and 3 had perioperative myocardial infarctions. Seventeen patients (41%) who initially improved required surgery within 6 months because of symptoms. Eleven patients (26%) were not operated on. It is concluded that acute coronary insufficiency is best managed initially by intensive medical therapy but a high proportion will require surgery later because of disabling angina. Early investigation and surgery are associated with a high mortality and incidence of myocardial infarction. Survivors of surgery are symptomatically improved and there is a low incidence of late infarction and death.