question: A retrospective observational study was conducted for patients 18 years or older presenting to a Midwestern emergency department (ED) in the United States during February 2019–January 2020 to characterize associated subsequent care utilization in patients who left the ED without being seen. Patients were classified as left without being seen (LWBS) based on documented ED disposition. The healthcare system’s records were reviewed for any associated utilizations within 3 weeks following the initial ED encounter. During the study period, 45,456 patients presented to the ED, with 2269 (5.0%) classified as LWBS. The median documented time until patients left the ED was 112 min. Of these patients, 1257 (55.4%) had a subsequent encounter within the health system within 3 weeks and 920 (73.2%) of these visits were determined to be related to the LWBS chief complaint. These visits included 67.5% of patients returning to ED or hospital, 27.5% to primary care or an urgent care clinic, and 5.0% to a specialty or other provider appointment. Of patients returning to ED, 78.1% did so within 72 h. Patients without a subsequent health system associated encounter tended to be younger, female, non-White, and present with possible lower-acuity chief complaints. At least one-half of LWBS patients sought care related to the concerns by a health system provider within 3 weeks of the initial encounter within the same system. The high prevalence of ED returns within a narrow turnaround window highlights a missed opportunity to provide services to these patients during their initial encounter. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11739-021-02775-z.