In 403 healthy subjects, 4 to 20 years of age, we measured total respiratory resistance (Rrs) and reactance (Xrs) between 4 and 26 Hz. When possible, vital capacity and forced expiratory volume in one second were also determined. The Rrs and Xrs vs frequency data depend on age or height, on sex, and slightly on weight. During growth, Rrs and the frequency dependence of Rrs decrease while Xrs increases. Only part of these changes may be explained entirely by a size effect. The variations with growth of ventilation unevenness, of airway wall compliance and of the topographic distribution of airway resistances probably also play a role. The influence of the interactions between height and age, and height and weight on Rrs and Xrs differ between sexes. Adult values for Rrs and Xrs are attained at the age of 15 years in girls and 18 years in boys.