In the critical care setting it may be difficult to determine an accurate reading of oxygen saturation from digital sensors as a result of poor peripheral perfusion. Limited evidence suggests that forehead sensors may be more accurate in these patients. We prospectively compared the accuracy of a forehead reflectance sensor (Max-Fast) with a conventional digital sensor in patients with acute respiratory distress syndrome during a high positive end-expiratory pressure (PEEP) recruitment manoeuvre (stepwise recruitment manoeuvre). Sixteen patients with early acute respiratory distress syndrome were enrolled to evaluate the blood oxygen saturation during a stepwise recruitment manoeuvre. PEEP was increased from baseline (range 10 to 18) to 40 cmH2O, then decreased to an optimal level determined by individual titration. Forehead and digital oxygen saturation and arterial blood gases were measured simultaneously before, during and after the stepwise recruitment manoeuvre at five time points. Seventy-three samples were included for analysis from 16 patients. The SaO2 values ranged from 73 to 99.6%. The forehead sensor provided measurements that deviated more from arterial measures than the finger sensor (mean absolute deviations 3.4%, 1.1% respectively, P=0.02). The greater variability in forehead measures taken at maximum PEEP was reflected in the unusually large precision estimates of 4.24% associated with these measures. No absolute differences from arterial measures taken at any other time points were significantly different. The finger sensor is as accurate as the forehead sensor in detecting changes in arterial oxygen saturation in adults with acute respiratory distress syndrome and it may be better at levels of high PEEP such as during recruitment manoeuvres.