To evaluate the operative risk of coronary diseased patients who had severe left ventricular dysfunction, we retrospectively reviewed the cases of 798 patients who had received A-C bypass surgery at Juntendo University between Jan. 1984 and Dec. 1989. The patients with severe left ventricular dysfunction (Ejection Fraction less than 30%) (Group-A, n = 9) were compared with the patients with moderately impaired left ventricular function (E.F. 30%-40%) (Group-B, n = 28) and normal left ventricular function (E.F. greater than or equal to 50%) (Group C, n = 34). The mean E.F. were 21.56 +/- 3.72% in Group-A, 34.28 +/- 3.17% in Group-B, 64.19 +/- 12.02% in Group-C. There were no differences between the 3 groups with regard to Cardiac Index, LVEDP, number of diseased vessels, number of grafts, aortic cross clamp time and cardio-pulmonary bypass time. The percentages of patient who needed catecholamines support in postoperatively were 77.8% in Group-A, 46.4% in Group-B and 61.8% in Group-C. There were no operative and hospital deaths in each of the 3 Groups, whereas 2 patients of Group-A died later of noncardiac disease. We propose that patients with severe left ventricular dysfunction (15 less than or equal to EF, less than 30%) benefit from CABG surgery with low risk, and that each left ventricular E.F., Cardiac Index or LVEDP alone were less significant predictors of operative results as measured by preoperative left ventricular function.