Coronary blood flow (CBF) and myocardial oxygen consumption (MVO 2 ), myocardial lactate extraction, blood gases, and hemodynamics were measured on and off intra-aortic balloon pumping (IABP) 14 times in 10 patients during treatment of cardiovascular collapse following myocardial infarction. At the time of study an average of 14 hours of IABP had elapsed and all but one patient had stabilized out of cardiogenic shock. With IABP, CBF fell in 7 instances, was unaffected in 3, and rose in 4. Changes in MVO 2 correlated closely with the observed effects on CBF. Lactate extraction and blood gases were not significantly affected. Systolic arterial pressure fell an average of 11 mm Hg with IABP, while mean diastolic pressure rose 8 mm Hg. These results suggest that the net result of IABP on coronary flow and metabolism depends on the interplay between increased blood flow to ischemic areas provided by increased diastolic perfusion pressure and diminution of blood flow to normal myocardium in which the oxygen requirements are reduced by diminished afterload.