Introduction: Delirium is a dreadful complication in seniors’ acute care. Many studies are available on the incidence of delirium, however ED-induced delirium is far less studied. We aim to evaluate the incidence and impact of ED-induced delirium among older non-delirious admitted ED patients who have prolonged ED stays (≥ 8 hours). Methods: This prospective INDEED study phase 1 included patients recruited from 4 Canadian EDs. Inclusion criteria: 1) Patients aged 65 and over; 2) ED stay ≥ 8 hours; 3) Patient is admitted to the hospital; 4) Patient is non-delirious upon arrival and at the end of the first 8 hours; 5) Independent or semi-independent patient. Eligible patients were assessed by a research assistant after an 8 hour exposition to the ED and evaluated twice a day up to 24h after ward admission. Patients’ functional and cognitive status were assessed using validated OARS and TICS-m tools. The Confusion Assessment Method was used to detect incident delirium. Hospital length of stays (LOS) were obtained. Univariate and multivariate analyses were conducted to evaluate outcomes. Results: Of the 380 patients prospectively followed, mean age was 76.5 (± 8.9), male represent 50% and 16.5% very old seniors (> 85 y.o.). The overall incidence of ED-induced delirium was 8.4%. Distribution by the 4 sites was: 10%, 13.8%, 5.5% & 13.4%. The mean ED LOS varied from 29 to 48 hours. The mean hospital LOS was increase by 6.1 days in the delirious patients compared to non-delirious patient (p<0.05). Increase mean hospital LOS distribution by site was by: 6.9, 8.5, 4.3 and 5.2 days for the ED-induced delirium patients. Conclusion: ED-induced delirium was recorded in nearly one senior out of ten after a minimal 8 hour exposure in the ED environment. An episode of delirium increases hospital LOS by about a week and therefore could contribute to ED overcrowding.