Introduction: Lung recruitment maneuvers followed by an individually titrated positive end-expiratory pressure (PEEP) are the key components of the open lung ventilation strategy in acute respiratory distress syndrome (ARDS). The staircase recruitment maneuver is a step-by-step increase in PEEP followed by a decremental PEEP trial. The duration of each step is usually 2minutes without physiologic rationale. Methods: In this prospective study, we measured the dynamic end-expiratory lung volume changes (EELV) during an increase and decrease in PEEP to determine the optimal duration for each step. PEEP was progressively increased from 5 to 40 cmH2O and then decreased from 40 to 5 cmH2O in steps of 5 cmH2O every 2.5minutes. The dynamic of EELV was measured by direct spirometry as the difference between inspiratory and expiratory tidal volumes over 2.5minutes following each increase and decrease in PEEP. EELV was separated between the expected increased volume, calculated as the product of the respiratory system compliance by the change in PEEP, and the additional volume. Results: Twenty-six early onset moderate or severe ARDS patients were included. Data are expressed as median [25th-75th quartiles]. During the increase in PEEP, the expected increased volume was achieved within 2[2-2] breaths. During the decrease in PEEP, the expected decreased volume was achieved within 1 [1-1] breath, and 95 of the additional decreased volume was achieved within 8 [2-15] breaths. Completion of volume changes in 99 of both increase and decrease in PEEP events required 29 breaths. Conclusions: In early ARDS, most of the EELV occurs within the first minute, and change is completed within 2minutes, following an increase or decrease in PEEP.