Interventions aimed at recruiting the lung of patients with acute respiratory distress syndrome (ARDS) are not uniformly effective. Because the prone position increases homogeneity of inflation of the lung, we reasoned that it might enhance its potential for recruitment. We ventilated 10 patients with early ARDS (PaO2/FIO2, 121 +/- 46 mm Hg; positive end-expiratory pressure, 14 +/- 3 cm H2O) in supine and prone, with and without the addition of three consecutive "sighs" per minute to recruit the lung. Inspired oxygen fraction, positive end-expiratory pressure, and minute ventilation were kept constant. Sighs increased PaO2 in both supine and prone (p < 0.01). The highest values of PaO2 (192 +/- 41 mm Hg) and end-expiratory lung volume (1840 +/- 790 ml) occurred with the addition of sighs in prone and remained significantly elevated 1 hour after discontinuation of the sighs. The increase in PaO2 associated with the sighs, both in supine and prone, correlated linearly with the respective increase of end-expiratory lung volume (r = 0.82, p < 0.001). We conclude that adding a recruitment maneuver such as cyclical sighs during ventilation in the prone position may provide optimal lung recruitment in the early stage of ARDS.