During proportional assist ventilation (PAV), resistive and elastic unloading relieve the work of breathing. Excessive unloading, however, results in resonant oscillations and runaway pressures. Our aim was to determine the appropriate levels of unloading that could be applied to clinical practice. A lung model, resistance (50 or 150 cmH(2)O l(-1) s(-1)) and compliance (0.4 or 0.8 ml/cmH(2)O), was used. The volumes and airway pressures delivered by the ventilator at varying levels of resistive and elastic unloading and simulated breaths were recorded. Oscillations in airway pressure only occurred when the level of resistive unloading exceeded the model's resistance. When the level of unloading fully compensated for the model's compliance, peak inflating pressures greater than 40 cmH(2)O were delivered; peak pressure limits of 20 cmH(2)O, however, resulted in very short (0.2 s or less) inflation times. High peak pressures were not delivered if the level of elastic unloading used was limited to that which reduced the model's elastance to that of a 'normal lung'. In conclusion, these results suggest that when using PAV, it is important to assess the compliance and resistance of the infant and endotracheal tube, so that levels of unloading that fully compensate for the resistance and compliance levels can be avoided.