Eighty patients with continued or repeated episodes of chest pain at rest and transitory ischemic electrocardiographic (ECG) changes were classified as having unstable angina pectoris. Following 10 days of intensive medical therapy, including beta blockade, all unerwent coronary arteriography. Medical treatment completely relieved the chest pain in 43 patients (Group I, 54 percent). In 37 patients (Group II, 46 percent) angina recurred within a week of admission (12 patients) or later (25 patients). Seventeen patients were not operated upon (nine were inoperable, four refused operation, and in four operation was not recommended). Sixty-three underwent saphenous vein bypass grafting either following a month of medical therapy (Group I) or within 24 to 48 hours of recurrent angina (Group II). The over-all operative mortality rate was 1.6 percent (1 patient) and the incidence of peroperative infarction was 11 percent. Of the 62 operative survivors, 71 percent were asymptomatic (mean follow-up period 22 months). The incidence of late operative myocardial infarction was 5 percent. Of 44 operative survivors tested by treadmill ECG, 66 percent had a negative response. Thirteen patients underwent postoperative angiographic evaluation (mean, 19.5 months). The over-all patency rate was 84 percent, and in 92 percent of patients at least one graft was patent. Thus, after stabilization by medical treatment, bypass operation could be performed with a low operative mortality rate and the long-term results compare favorably with those achieved with chronic stable angina.