IntroductionThe number of emergency department observation units (EDOU) and observation stays has continued to increase. Despite this, there is limited data on the characteristics of patients who return unexpectedly to the ED after EDOU discharge.MethodsWe identified the charts of all patients who were admitted to the EDOU of an academic medical center between January 2018–June 2020 and had a return to the ED within 14 days of discharge from the EDOU. Patients were excluded if they were admitted to the hospital from the EDOU, left against medical advice, or died in the EDOU. We manually extracted selected demographic factors, comorbidities, and healthcare utilization data from the charts. Physician reviewers identified return visits thought to be related to the index visit or potentially avoidable.ResultsDuring the study period, there were 176,471 ED visits, 4,179 admissions to the EDOU, and 333 return visits to the ED within 14 days from discharge from the EDOU, representing 9.4% of all patients discharged from the EDOU. We identified a higher rate of return for patients treated for asthma and lower rates of return for patients treated for chest pain or syncope than the overall return rate. Physician reviewers determined that 64.6% of unplanned returns were related to the index visit, and 4.5% were potentially avoidable. Of potentially avoidable visits, 53.3% occurred within 48 hours of discharge, supporting the use of this period as a potential quality metric. While there was no significant difference in the percentage of related return visits between males and females, there was a higher rate of potentially avoidable visits for male patients.ConclusionThis study adds to the limited body of literature on EDOU returns, finding an overall return rate of under 10%, with about two-thirds of returns determined to be related to the index visit and <5% considered to be potentially avoidable.