OBJECTIVE Data from a national registry of 23 centers using cardiopulmonary support (CPS) were analyzed to compare the risks and benefits of prophylactic CPS versus standby CPS for patients undergoing high risk coronary angioplasty. BACKGROUND Early data from the CPS registry documented a high angioplasty success rate as well as a high procedural morbidity rate. Because of this increased morbidity some high risk patients were placed on standby CPS instead of prophylactic CPS. METHODS Patients in the prophylactic CPS group had 18F or 20F venous and arterial cannulas inserted and cardiopulmonary bypass initiated. Patients in the standby CPS group were prepared for institution of cardiopulmonary bypass, but bypass was not actually initiated unless the patient sustained irreversible hemodynamic compromise. RESULTS There were 389 patients in the prophylactic CPS group and 180 in the standby CPS group. The groups were comparable with respect to most baseline characteristics, except that left ventricular ejection fraction was lower in the prophylactic CPS group. Thirteen of the 180 patients in the standby CPS group sustained irreversible hemodynamic compromise during the angioplasty procedure. Emergency institution of CPS was successfully initiated in 12 of these 13 patients in < 5 min. Procedural success was 88.7% for the prophylactic and 84.4% for the standby CPS group (p = NS). Major complications did not differ between groups. However, 42% of patients in the prophylactic CPS group sustained femoral access site complications or required blood transfusions, compared with only 11.7% of patients in the standby CPS group (p < 0.01). Among patients with an ejection fraction < or = 20%, procedural morbidity remained significantly higher in the prophylactic CPS group (41% vs. 9.4%, p < 0.01), but procedural mortality was higher in the standby group (4.8% vs. 18.8%, p < 0.05). CONCLUSIONS Patients in the standby and prophylactic CPS groups had comparable success and major complication rates, but procedural morbidity was higher in the prophylactic group. When required, standby CPS established immediate hemodynamic support during most angioplasty complications. For most patients, standby CPS was preferable to prophylactic CPS during high risk coronary angioplasty. However, patients with extremely depressed left ventricular function (ejection fraction < 20%) may benefit from institution of prophylactic CPS.