To identify high-risk subgroups, 468 patients with unstable angina were prospectively stratified according to the clinical presentation of unstable angina (type I or type II) and left ventricular function (normal or abnormal) and were randomized to conventional medical therapy or surgical treatment with coronary bypass surgery. Type I patients (n = 374) were those who had progressive effort angina or recent angina at rest. Type II patients (n = 94) were those who had severe rest angina associated with ST-T changes on the electrocardiogram. Follow-up for 8 years showed that the cumulative mortality rates for type II patients with abnormal left ventricular function were significantly lower in the surgical patients compared with the medical cohorts (13% versus 46%, p less than 0.04). In the other subgroups, cumulative medical and surgical mortality rates were not different. Thus, type II patients with abnormal left ventricular function appear to be the subgroup of patients who are at the highest risk with medical therapy. Coronary bypass surgery significantly reduces the mortality in this high-risk subgroup of patients with unstable angina.