question: OBJECTIVES The coronavirus disease 2019 pandemic has strained many healthcare systems. In response, U.S. hospitals altered their care delivery systems, but there are few data regarding specific structural changes. Understanding these changes is important to guide interpretation of outcomes and inform pandemic preparedness. We sought to characterize emergency responses across hospitals in the United States over time and in the context of local case rates early in the coronavirus disease 2019 pandemic. DESIGN We surveyed hospitals from a national acute care trials group regarding operational and structural changes made in response to the coronavirus disease 2019 pandemic from January to August 2020. We collected prepandemic characteristics and changes to hospital system, space, staffing, and equipment during the pandemic. We compared the timing of these changes with county-level coronavirus disease 2019 case rates. SETTING AND PARTICIPANTS U.S. hospitals participating in the Prevention and Early Treatment of Acute Lung Injury Network Coronavirus Disease 2019 Observational study. Site investigators at each hospital collected local data. INTERVENTIONS None. MEASUREMENTS AND MAIN RESULTS Forty-five sites participated (94% response rate). System-level changes (incident command activation and elective procedure cancellation) occurred at nearly all sites, preceding rises in local case rates. The peak inpatient census during the pandemic was greater than the prior hospital bed capacity in 57% of sites with notable regional variation. Nearly half (49%) expanded ward capacity, and 63% expanded ICU capacity, with nearly all bed expansion achieved through repurposing of clinical spaces. Two-thirds of sites adapted staffing to care for patients with coronavirus disease 2019, with 48% implementing tiered staffing models, 49% adding temporary physicians, nurses, or respiratory therapists, and 30% changing the ratios of physicians or nurses to patients. CONCLUSIONS The coronavirus disease 2019 pandemic prompted widespread system-level changes, but front-line clinical care varied widely according to specific hospital needs and infrastructure. Linking operational changes to care delivery processes is a necessary step to understand the impact of the coronavirus disease 2019 pandemic on patient outcomes.