Intra-aortic balloon pumping is effective in reducing left ventricular peak systolic pressure and increasing cardiac output in patients with severe cardiogenic shock secondary to acute myocardial infarction. Associated with these effects is a reduction in intravenous catecholamine requirements and increased sensitivity to diuretics. Reduction in left ventricular end-diastolic pressure is implied by improvement in arterial blood gas saturation (reduced arterioalveolar gradient) and was shown directly in one patient. Total myocardial oxygen consumption is probably reduced by counterpulsation. Platelet levels have not fallen significantly with adequate heparinization and the slow infusion of low molecular weight dextran. No complications have occurred related to femoral artery cannulation, nor has there been significant damage to the aorta. The findings suggest that this system is a safe means of assisting the failing left ventricle. The high mortality in this series (seven of eight patients) is related to the extent of irreversible myocardial damage which may be reduced by more aggressive, earlier application of counterpulsation.