Key PointsQuestionWhat are the associated regional health care disruptions in hospitals adjacent to health care systems under ransomware cyberattack?FindingsThis cohort study of 2 academic urban emergency departments (EDs) adjacent to a health care delivery organization under a month-long ransomware attack evaluated 19 857 ED visits at the unaffected ED: 6114 in the preattack phase, 7039 in the attack and recovery phase, and 6704 in the postattack phase. During the attack and postattack phases, significant increases in patient census, ambulance arrivals, waiting room times, patients left without being seen, total patient length of stay, county-wide emergency medical services diversion, and acute stroke care metrics were seen in the unaffected ED.MeaningThis study suggests that health care cyberattacks such as ransomware are associated with greater disruptions to regional hospitals and should be treated as disasters, necessitating coordinated planning and response efforts.