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To investigate respiratory and hemodynamic changes during lung recruitment and descending optimal positive end-expiratory pressure (PEEP) titration.Prospective auto-control clinical trial.Adult general intensive care unit in a university hospital.Eighteen patients with acute respiratory distress syndrome.Following baseline measurements (T0), PEEP was set at 26 cm H2O and lung recruitment was performed (40/40-maneuver). Then tidal volume was set at 4 mL/kg (T26R) and PEEP was lowered by 2 cm H2O in every 4 mins. Optimal PEEP was defined at 2 cm H2O above the PEEP where Pao2 dropped by10%. After setting the optimal PEEP, the 40/40-maneuver was repeated and tidal volume set at 6 mL/kg (T(end)).Arterial blood gas analysis was done every 4 mins and hemodynamic measurements every 8 mins until T(end), then in 30 (T30) and 60 (T60) mins. The Pao2 increased from T0 to T(end) (203 +/- 108 vs. 322 +/- 101 mm Hg, p.001), but the extravascular lung water (EVLW) did not change significantly. Cardiac index (CI) and the intrathoracic blood volume (ITBV) decreased from T0 to T26R (CI, 3.90 +/- 1.04 vs. 3.62 +/- 0.91 L/min/m2, p.05; ITBVI, 832 +/- 205 vs. 795 +/- 188 m/m2, p.05). There was a positive correlation between CI and ITBVI (r = .699, p.01), a negative correlation between CI and central venous pressure (r = -.294, p.01), and no correlation between CI and mean arterial pressure (MAP).Following lung recruitment and descending optimal PEEP titration, the Pao2 improves significantly, without any change in the EVLW up to 1 hr. This suggests a decrease in atelectasis as a result of recruitment rather than a reduction of EVLW. There is a significant change in CI during the maneuver, but neither central venous pressure, heart rate, nor MAP can reflect these changes.