Using end-inspiratory airway occlusion, respiratory system resistance (Rrs) can be partitioned into a flow-resistive component (Rint), and an additional component (DeltaR), reflecting viscoelasticity and time constant inequalities. We studied flow and volume dependence of Rrs and its subdivisions (Rint and DeltaR) in 13 children, seven mechanically ventilated for pulmonary insufficiency (Group 1; six with parenchymal lung disease; one with lower airway obstruction) and six without primary lung disorder (Group 2). In comparison with healthy children, Rint was increased in the patient with lower airway obstruction and five of six patients without primary lung disorder but in only one of six with parenchymal lung disease. DeltaR was increased in all seven patients in Group 1 and in four of six patients in Group 2. The directions of changes in Rint and Rrs with increasing flow (isovolume conditions) and with increasing volume (isoflow conditions) were variable. DeltaR decreased exponentially (p < 0.05) with increasing flow in 11 of 13 subjects and increased with increasing tidal volume (VT) in 12 of 13. Thus, DeltaR was increased in most children on mechanical ventilation with or without primary lung disease; its volume and flow dependence were opposite to that of airway resistance.