The proposed study is a randomized, crossover trial. We plan to enroll 40 patients with ALI and randomize to APRV or conventional MV for 24 hours. After this time the patients will be switched to the alternative mode of ventilation (MV or APRV) for another 24 hours. To assess breathing comfort, at the end of each 24-hour period we will measure the amounts of sedative and analgesic medications used. We will also measure the concentrations of markers of inflammation in the blood and lung as measures of VILI. Finally, throughout the study we will compare the adequacy of gas exchange with APRV compared to conventional MV.