Maintenance of spontaneous breathing superimposed on mechanical ventilation is suggested to improve gas exchange in patients with acute lung injury. The aim of this study was to evaluate the long-term effects of airway pressure release ventilation with maintained unsupported spontaneous breathing (APRV) and synchronized intermittent mandatory ventilation with pressure support (SIMV) on the amount of lung collapse in acute lung injury patients. Thirty-seven patients with acute lung injury were studied in a trial comparing APRV or SIMV. Computer-assisted tomography scannings (CT) were performed before randomization and at day 7. The change in the amount of nonaerated lung was comparable between groups; 14.7% (3.8-17.4) in APRV group (n = 13) and 9.6% (-1.4 to 18.62) in the SIMV group (n = 10), (P = .65, difference in mean 4.9%, 95% confidence interval -9.0% to 19.0%). The effects of APRV and SIMV on lung aeration are similar after 7 days of mechanical ventilation.