A nonlinear model of breathing mechanics, in which the tracheobronchial airways are considered in three serial segments, is presented to obtain insights into the mechanisms underlying expiratory flow limitation (EFL) in mechanically ventilated patients. Chronic obstructive pulmonary disease (COPD) and normal conditions were simulated and EFL was detected by application of negative expiratory pressure at the mouth or resistance reduction of the expiratory circuit. Simulation results confirm that both techniques reveal remarkable differences in the flow-volume curves between normal subjects and COPD patients, the former showing absence of EFL and the latter exhibiting EFL over most of the expiration. To interpret the role of different nonlinear mechanisms in producing EFL, different flow-volume curves obtained by changing model parameter values were analyzed. An increase in lower-airway resistance did not give rise to EFL, whereas a change in the pressure-volume characteristic of the intermediate-airway segment, towards increased resistance and easier collapse, significantly modified system behavior. In particular, EFL was observed when this intermediate-segment change was combined with an increase in lower-airway resistance. This evidence suggests that modifications, producing loss of radial traction and consequent narrowing of the airways in the peribronchial region, may play a leading role in EFL in COPD patients.