The ARDSNet trial revealed that the use of a smaller tidal volume (V_T) reduced mortality by 22%. However, three earlier studies that lowered V_T did not find a decrease in mortality. We tested the hypothesis that the increased respiratory rate used in the ARDSNet lower V_T strategy might have led to intrinsic positive end-expiratory pressure (PEEP_i), raising total PEEP (PEEP_total). Ten patients with acute respiratory distress syndrome (ARDS) were ventilated using the ARDSNet lower V_T protocol. Respiratory rate was then reduced (10–15 breaths/minute) to obtain a V_T of 12 ml/kg (ARDSNet traditional V_T). PEEP on the ventilator (PEEP_nominal: 10.1 ± 0.7 cm H₂O), Fi_O2 (0.7 ± 0.1), and minute ventilation (V_E: 12.4 ± 1.7 L/minute) were set using the ARDSNet protocol and maintained constant during the two ventilatory strategies. Values of airway pressure at end-expiration of a regular breath (PEEP_external) and 3–5 seconds after the onset of an end-expiratory occlusion (PEEP_total) were measured. PEEP_i was calculated by subtracting PEEP_external from PEEP_total. PEEP_total and PEEP_i were, respectively, 16.3 ± 2.9 and 5.8 ± 3.0 cm H₂O during the lower V_T strategy and 11.7 ± 0.9 and 1.4 ± 1.0 cm H₂O during the traditional V_T strategy (p < 0.01). The reduced mortality observed with the ARDSNet strategy may have been due to the protective effect of a higher PEEP_total.