This prospective un-blinded randomized trial will follow patients with respiratory failure and ALI/ARDS who require invasive mechanical ventilation in select ICUs. Patients will be allocated to respiratory support with either APRV mode or volume control (AC) mode of mechanical ventilation. Qualifying patients will be randomized by permuted block randomization within 24 hours of admission to the ICU. Identical ventilation and oxygenation thresholds will be utilized to guide titration of each ventilator protocol. Patients will remain on the assigned mode of ventilation until they are extubated and discharged from the ICU.