Copyright ©ERS Journals Ltd 1998 European Respiratory Journal

Reversibility of airway obstruction is considered one of the major criteria in the diagnosis of asthma in children, adolescents and adults [1, 2]. Different guidelines for the cut-off value have been proposed for forced expiratory vol-ume in one second (FEV1) and airway resistance (Raw), expressed as either % baseline or as % predicted [3–10]. Spirometry is hard to perform in children younger than 6 yrs old. Normal values of Raw [11] and of total respira-tory resistance (Rrs) measured with the forced oscillation technique (FOT) [12, 13] have been established in chil-dren [14–16]. Not many data are available on bronchodi-lation tests in asthmatic children younger than 6 yrs. In some of these studies the impulse oscillation system (IOS) was used [17, 18], a FOT involving the application of a rectangular pulse signal, whilst in the FOT a pseudoran