Inhaled nitric oxide (NO) and prone position (PP) are frequently used in the treatment of acute respiratory distress syndrome (ARDS). We compared the gas exchange and hemodynamic effects induced by the combination of NO inhalation and PP in patients with ARDS and analyzed whether or not pulmonary (Pu) and extrapulmonary (Epu) ARDS patients behave differently. Eight Pu and seven Epu ARDS patients were studied in four situations: supine position (SP); SP with NO inhalation at 5 ppm (SP + NO); PP; and PP with NO inhalation (PP + NO). In comparison with SP, NO inhalation and PP induced significant increases in Pa(O(2))/FI(O(2)) (from 106 +/- 58 in SP to 131 +/- 69 mm Hg in SP + NO, p = 0.01, and to 184 +/- 67 mm Hg in PP, p < 0.001). Pu and Epu ARDS showed a similar improvement in Pa(O(2))/FI(O2) with PP. Only Pu ARDS patients showed a significant increase (p < 0.001) in oxygenation induced by NO inhalation from 81 +/- 45 to 100 +/- 50 mm Hg in SP, and from 146 +/- 53 to 197 +/- 98 mm Hg in PP. In conclusion, PP is associated with a marked improvement in oxygenation, irrespective of the causes of ARDS, and additive effects of NO inhalation are mainly seen in patients with Pu ARDS.