Optimization of patients flow can decrease the overcrowding of the emergency department (ED) and improve the level of patient's satisfaction. The prospective analysis of 502 consecutive admissions at the Nyon regional hospital's ED shows a mean length of stay (LOS) of 145 minutes, for a majority of low-severity cases admitted during working hours. A significant LOS variability reveals transient episodes of overcrowding, which lead to an increase in the LOS of mainly ambulatory patients. Our analysis shows that both the ED's structure and the medical organization create a gap between ED performance as care provider and the patients' expectations, and suggests that a separation of low-versus high-complexity cases is necessary. Patients' flow analysis can detect ED overcrowding and may help find the appropriate solutions to reduce it.