High-occupancy hospitals may be sensitive to neighboring emergency department (ED) closures and openings, as they already operate at or near capacity. We conducted a retrospective analysis using data for the period 2001-13 to examine outcomes of and treatment received by patients with acute myocardial infarction at so-called bystander EDs that had been exposed to nearby ED closures or openings. We used changes in driving time between an ED and the next-closest one as a proxy for a closure or opening: If driving time increased, for instance, it meant that a nearby ED had closed. When a high-occupancy ED was exposed to a closure that resulted in increased driving time of thirty minutes or more to the next-closest ED, one-year mortality and thirty-day readmission rates increased by 2.39 and 2.00 percentage points, respectively, while the likelihood of receiving percutaneous coronary intervention (PCI) declined by 2.06 percentage points. Exposure to ED openings that resulted in decreased driving times of thirty minutes or more was associated with reductions in thirty-day mortality at bystander hospitals and an increased likelihood of receiving PCI. Our findings suggest that limited resources at high-occupancy bystander hospitals make them sensitive to changes in the availability of emergency care in neighboring communities.