BACKGROUND We wanted to assess whether there was a significant relationship between recruited lung volume (V(rec)) and change in density on digital processed chest x-ray measured at 2 different levels of inspiratory plateau pressure corresponding to 2 PEEP levels in patients with acute lung injury or ARDS. METHODS In 14 subjects, PEEP 5 cm H2O and 15 cm H2O were prospectively applied in a random order for 10 min. At the end of each period, chest x-ray was taken using a digital portable device, and a pressure-volume curve of the respiratory system was performed. We also assessed P(aO2), and the static and the dynamic (C(dyn,rs)) compliance of the respiratory system. Change in end-expiratory lung volume between tidal breath and relaxation volume of the respiratory system was determined. Radiological attenuation was measured on chest x-rays in 4 regions of interest in the right lung, and in 3 regions of interest in the left lung, drawn in posterior intercostal spaces from top to bottom, by using dedicated software. The ratio of lung density in each region between PEEP 15 and PEEP 5 (rP15/P5) and their arithmetic mean (μP15/P5) were computed. V(rec) was determined from the pressure-volume curves. RESULTS The median value of rP15/P5 in the 98 lung levels was 0.91 (0.80-1.01), which was significantly different from 1 (P < .001). The values of rP15/P5 were not significantly different between the lung levels. The median values of V(rec) and μP15/P5 were 288 (173-402) mL and 0.90 (0.80-0.97), respectively. There was a significant negative correlation between V(rec) and μP15/P5 (R = -0.77, P = .01). The reduction in μP15/P5 tended to correlate with the increase in C(dyn,rs) (R = -0.49, P = .077) or in P(aO2) (R = -0.53, P = .05) between PEEP 15 cm H2O and PEEP 5 cm H2O. CONCLUSIONS Digital chest x-ray done at the bedside in acute lung injury/ARDS subjects was able to detect a reduction in density between PEEP 5 cm H2O and PEEP 15 cm H2O, which correlated with V(rec).