Aortocoronary bypass surgery in patients with left main coronary artery disease is reported to have an operative mortality of between 1.4 and 39%. It is generally accepted that the operative mortality in this group of patients is considerably greater than in routine bypass candidates, presumably due to the large amount of myocardium threatened by a single lesion. In an effort to preserve threatened left ventricular myocardium, intra-aortic balloon pumping was instituted prophylactically prior to sternotomy in 20 consecutive patients with left main coronary artery disease (luminal narrowing greater than 50%). Sixty per cent of these patients had New York Heart Association Class IV angina, 25% had Class III, and 15% Class II. Fifty per cent of the patients in this group presented with unstable angina. Operative patients requiring left ventricular aneurysmectomy and/or valve replacement, were excluded. No operative deaths have been encountered in 20 consecutive patients managed in this manner. One patient displayed signs of myocardial infarction in the postoperative period. Correctable peripheral vascular ischemic complications of pump insertion were encountered in three patients. Preliminary results from this ongoing study support the hypothesis that prophylactic intra-aortic balloon pumping is a low risk procedure that should be utilized routinely in aortocoronary bypass surgery for left main coronary artery disease.