Body fluid status is one of the most important parameters estimated during every clinical examination. In many disorders, the therapy and its effectiveness depend on appropriate evaluation of body water status. There are some useful methods of evaluating body fluid incontinence; however, all of them are burdened with some limitations, especially when used in the emergency department. We introduce a new sonographic parameter, the "inferior vena cava (IVC)/aorta (Ao) diameter (IVC/Ao) index," as a convenient, quick, and effective way of evaluating body fluid status. The usefulness of the sonographic assessment of IVC diameter has already been proven in children with nephrotic syndrome, in hemodialysis patients, and in patients hospitalized in intensive care units. However, this method is limited by the necessity of comparing IVC diameter to body surface, measuring IVC diameter during maximal inspiration and expiration, or lack of reference values for the pediatric population, which can be eliminated in our new method of body fluid status assessment.