The medical part of the activity in the emergency units increased and has been ascribed to the raised number of infants or children admitted for acute dyspnea. This review is based on the published reports and the experience from the paediatric emergency unit from the Rouen area with 450,000 inhabitants, in France. We put forward the known epidemiological data and discussed the available means for the practitioner, which could help him in the decision to hospitalize. This review suggests that algorithms of treatment and severity evaluations must be set up in paediatric emergency units in order to validate them and specify the children who requires hospitalisation.