Unidirectional intra-aortic balloon pumping (IABP) was applied to 28 adult patients undergoing open-heart surgery over a 35 month period. The patients were divided into three groups according to the temporal sequence of initiating IABP. Group A consisted of 4 patients who were in a low output state or in cardiogenic shock prior to study. All patients survived cardiac catheterization and surgery, and 3 (75 per cent) were long-term survivors. Group B included 15 patients who could not be weaned from cardiopulmonary bypass with the usual supportive measures. Twelve patients (80 per cent) were weaned from bypass with IABP, and 11 (73 per cent) were discharged from the hospital. Group C was composed of 9 patients who manifested a low cardiac output syndrome within the first 24 hours following surgery. IABP was initiated in the recovery room. Six patients (67 per cent) were discharged. The total experience with these 28 patients therefore includes 24 patients (86 per cent) who were weaned from IABP, 20 (71 per cent) who were discharged, and 18 (64 per cent) who were long-term survivors. The present criteria for the use of IABP in the cardiac surgical patient are defined.