To assess whether prolonged length of stay in the emergency department was associated with risk of death. We analysed data from 165,183 arrivals at St. Olav’s University Hospital’s emergency department from 2011 to 2018, using an instrumental variable method. As instruments for prolonged length of emergency department stay, we used indicators measured before arrival of the patient. These indicators were used to study the association between prolonged length of emergency department stay and risk of death, being discharged from the emergency department and length of hospitalisation for those who were hospitalised. Mean length of stay in the emergency department was 2.9 hours, and 30-day risk of death was 3.4%. Per hour prolonged length of stay in the emergency department, the overall change in risk of death was close to zero, with a narrow 95% confidence interval of −0.5 to 0.7 percentage points. Prolonged emergency department stay was associated with a higher probability of being discharged from the emergency department without admission to the hospital. We found no substantial differences in length of hospitalisation for patients who were admitted. In this study, prolonged emergency department stay was not associated with increased risk of death. This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.