Pulmonary function tests (PFT) are usually useful to confirm the diagnosis, as well as for guidance to therapy and also to clarify the prognosis of several respiratory diseases, but only recently it gained interest in preschool aged children. Published papers have shown the feasibility of respiratory function testing in preschool aged children, as in the context of clinical investigation, as in clinical daily practice, but more investigation is needed to demonstrate PFT usefulness, particularly in clinical correlation and prospective follow-up of preschool asthmatic children.