Background Non-infarct-related artery (non-IRA) disease is prevalent in patients with ST-segment elevation myocardial infarction (STEMI). We aimed to assess the impact of non-IRA disease on infarct size and clinical outcomes in patients with acute STEMI. Methods The Counterpulsation to Reduce Infarct Size Pre-PCI Acute Myocardial Infarction (CRISP-AMI) trial randomized patients to intra-aortic balloon counterpulsation (IABC) vs no IABC prior to percutaneous coronary intervention in patients with acute STEMI. Infarct size (% left ventricular mass) at 3-5 days post percutaneous coronary intervention and 6-month clinical outcomes were compared between patients with and without non-IRA disease (defined as ≥50% stenosis in at least one non-IRA). Results A total of 324 (96.1%) patients had anterior STEMI, of whom 34.9% had non-IRA disease. There was no difference in infarct size (% left ventricular mass) between patients with and without non-IRA disease (median 39% vs 39%; P = .73). At 6 months, there was no difference in rates of recurrent myocardial infarction (0.9% vs 0.9%; P = .78), major Thrombolysis In Myocardial Infarction bleeding (0.9% vs 0.5%; P = .77), or all-cause death (3.5% vs 2.4%; P = .61) in patients with and without non-IRA disease, respectively. Patients with non-IRA disease had a higher rate of new/worsening heart failure with hospitalization (8.8% vs 1.9%; P = .0050). Conclusions More than one-third of patients with anterior STEMI in the CRISP-AMI study had non-IRA disease. These patients had similar infarct sizes and rates of recurrent myocardial infarction, major bleeding, and all-cause death. Patients with non-IRA disease did have a higher rate of new/worsening heart failure with hospitalization. Further study is needed to understand the mechanisms of outcomes of patients with non-IRA disease.