Background Despite advances in primary percutaneous coronary intervention (pPCI) and regional systems of care, the development of cardiogenic shock is associated with poor clinical outcomes in patients with ST-segment elevation myocardial infarction (STEMI). We sought to better characterize the baseline characteristics and clinical outcomes of patients who underwent crossover to intraaortic balloon counterpulsation (IABC) in the CRISP AMI trial. Methods Patients with anterior STEMI were randomized to IABC before pPCI or pPCI alone. Infarct size and 6-month clinical outcomes were evaluated in patients both in the pPCI-alone group who did undergo crossover to IABC and those who did not undergo crossover to IABC. Results Among 176 patients randomized to pPCI alone, 161 patients did not later receive IABC during the index hospitalization, and 15 patients (8.5%) underwent crossover and did receive unplanned IABC. Hypotension and/or cardiogenic shock precipitated crossover to IABC in 12 patients (80%). Patients who underwent crossover to IABC demonstrated lower systolic and diastolic blood pressures on admission. At 6 months, rates of death (26.7% vs 3.1%, P =.003), readmission for severe hypotension (53.3% vs 3.7%, P <.001), resuscitated cardiac arrest, and ventricular arrhythmia were higher in the group that did crossover to IABC. Crossover to IABC was not associated with increased infarct size. Conclusions The most significant predictor of crossover to IABC in the setting of anterior STEMI was relative hypotension at the time of hospital admission, and crossover to IABC in CRISP AMI was associated with significantly worse clinical outcomes.