BACKGROUND: With the improvement in outcomes for acute coronary syndrome (ACS), the practice of routine admission to cardiac intensive care units (CICUs) is evolving. We aimed to describe the epidemiology of patients with ACS admitted to contemporary CICUs. METHODS: Using the CCCTN (Critical Care Cardiology Trials Network) Registry for consecutive medical CICU admissions across 26 advanced CICUs in North America between 2017 and 2020, we identified patients with a primary diagnosis of ACS at CICU admission and compared patient characteristics, resource utilization, and outcomes to patients admitted with a non-ACS diagnosis and across sub-populations of patients with ACS, including by indication for CICU admission. RESULTS: Of 10 118 CICU admissions, 29.4% (n=2978) were for a primary diagnosis of ACS, with significant interhospital variability (range, 13.4%-56.6%). Compared with patients admitted with a diagnosis other than ACS, patients with ACS had fewer comorbidities, lower acute severity of illness with less utilization of advanced CICU therapies (41.3% versus 66.1%, P<0.0001), and lower CICU mortality (5.4% versus 9.9%, P<0.0001). Monitoring alone, without another CICU indication at the time of admission, was the most frequent admission indication in patients with ACS (53.8%); less common indications in patients with ACS included respiratory insufficiency, shock, or the need for vasoactive therapy. Of patients with ACS admitted for monitoring alone, 94.8% did not subsequently require advanced intensive care unit therapies and had a low CICU length of stay (1.5 days [0.9-2.4] versus 2.6 [1.4-5.1], P<0.0001) and CICU mortality (0.6% versus 11.0%, P<0.0001), compared with patients with ACS with an admission indication beyond monitoring. CONCLUSIONS: In a registry of tertiary care CICUs, ACS represent ≈1/3 of all admissions with significant variability across hospitals. More than half of the ACS admissions to the CICU were for routine monitoring alone, with a low rate of complications and mortality. This observation highlights an opportunity for prospective studies to refine triage strategies for lower risk patients with ACS.