Intermittent Mandatory Pressure Release Ventilation (IMPRV) is a positive pressure spontaneous breathing ventilatory mode in which airway pressure is released intermittently and synchronously with patient's spontaneous expiration in order to provide ventilatory assistance. Eight critically ill patients free of any factor known to alter chest wall mechanics (group 1) and 8 critically ill patients whose spontaneous respiratory activity was markedly altered by a flail chest, or by a C₅ quadraplegia and/or by the administration of opioids (group 2) were studied prospectively. CPAP and IMPRV were administered to each patient in a random order during a 1 h period using a CESAR ventilator. Gas flow, tidal volume, tracheal pressure, esophageal pressure, end-expiratory lung volume and hemodynamic parameters were measured. In group 1 patients, the ventilatory assistance provided by IMPRV was associated with a significant decrease in spontaneous tidal volume whereas all other respiratory parameters remained unchanged. In group 2 patients, IMPRV increased minute ventilation from 8.0±2.61/min to 12.2±1.81/min (p<0.05), decreased PaCO₂ from 46±7.3 mmHg to 38±6.8 mmHg (p<0.05) and reduced respiratory frequency from 21±10 bpm to 14±5.7 bpm (p<0.07). These results show that IMPRV provides significant ventilatory assistance to patients with mild acute respiratory failure either by decreasing patient's contribution to minute ventilation or by increasing alveolar ventilation in presence of respiratory depression of central or peripheral origin.