STUDY OBJECTIVE: Monitor alarms are prevalent in the ED. Continuous electronic monitoring of patients' vital signs may alert staff to physiologic decompensation. However, repeated false alarms may lead to desensitization of staff to alarms. Mitigating this could involve prioritizing the most clinically-important alarms. There are, however, little data on which ED monitor alarms are clinical meaningful. We evaluated whether and which ED monitor alarms led to observable changes in patients' ED care. METHODS: This prospective, observational study was conducted in an urban, academic ED. An ED physician completed 53 h of observation, recording patient characteristics, alarm type, staff response, whether the alarm was likely real or false, and whether it changed clinical management. The primary outcome was whether the alarm led to an observable change in patient management. Secondary outcomes included the type of alarms and staff responses to alarms. RESULTS: There were 1049 alarms associated with 146 patients, for a median of 18 alarms per hour of observation. The median number of alarms per patient was 4 (interquartile range 2-8). Alarms changed clinical management in 8 out of 1049 observed alarms (0.8%, 95% CI, 0.3%, 1.3%) in 5 out of the 146 patients (3%, 95% CI, 0.2%, 5.8%). Staff did not observably respond to most alarms (63%). CONCLUSION: Most ED monitor alarms did not observably affect patient care. Efforts at improving the clinical significance of alarms could focus on widening alarm thresholds, customizing alarms parameters for patients' clinical status, and on utilizing monitoring more selectively.