Background The intraaortic balloon pump has been shown to improve cardiac output and diastolic coronary flow. Animal studies with balloon counterpulsation have shown variable effects on carotid and cerebral blood flow. We investigated the effects of counterpulsation with the intraaortic balloon pump on blood flow in the common carotid artery in human beings. Methods and Results We studied 14 patients who had an intraaortic balloon pump placed for clinical indications; 9 were hypotensive (4 in the setting of an acute myocardial infarction and 5 immediately after cardiac surgery). Five patients required counterpulsation after undergoing complicated coronary angioplasty. Common carotid artery blood flow was assessed with carotid Duplex scanning both with and without 1:1 balloon augmentation. We found no net increase in the total blood flow in the common carotid artery with intraaortic balloon counterpulsation. This result occurred despite a significant mean increase in both the peak flow velocity and flow velocity integral of the augmented diastolic flows by 160% and 78%, respectively. Total flow did not change because an early systolic reversal of blood flow was seen only with balloon augmentation, which negated the augmented diastolic flow. Conclusion Despite a significant augmentation in diastolic blood flow with balloon counterpulsation, no increase occurred in total carotid artery blood flow. This appeared to be caused by improper timing of balloon deflation. This could result in no improvement in cerebral blood flow with intraaortic balloon pump counterpulsation in critically ill patients. (Am Heart J 1998;135:850-4.)