The forced pseudo-random noise oscillation technique is a method by which total respiratory resistance (Rrs) and reactance (Xrs) can be measured simultaneously at various frequencies by means of complex oscillations, superimposed at the mouth during spontaneous quiet breathing. Reference values were obtained in 255 healthy Caucasian children of Dutch descent aged 2.3-12.5 years. Rrs and Xrs vs frequency (f) curves are mainly determined by the child's sex, age, height and weight. Taking complete Rrs and Xrs-f curves into account, we found that Rrs values were significantly higher in young boys than in young girls. They were equal at about 8 years, but at about 12 years of age Rrs values were again significantly higher in boys than in girls. Frequency dependence of Rrs was found in healthy boys up to about 5 years of age, but not in girls of the same age or in older children. These data suggest differences in airway diameter between boys and girls. At all ages Xrs was significantly lower in boys than in girls. This suggests differences in bronchial patency of peripheral airways, boys being at a disadvantage. It is concluded that multiple frequency oscillometry is a method which is ideal for children from the age of about 3 years. The possibility of measuring Rrs as well as frequency dependence of Rrs and Xrs simultaneously is the major advantage over other oscillation devices.