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Purpose The PaO.sub.2/FiO.sub.2 is an integral part of the assessment of patients with acute respiratory distress syndrome (ARDS). The American-European Consensus Conference definition does not mandate any standardization procedure. We hypothesized that the use of PaO.sub.2/FiO.sub.2 calculated under a standard ventilatory setting within 24 h of ARDS diagnosis allows a more clinically relevant ARDS classification. Methods We studied 452 ARDS patients enrolled prospectively in two independent, multicenter cohorts treated with protective mechanical ventilation. At the time of ARDS diagnosis, patients had a PaO.sub.2/FiO.sub.2 [less than or equal to] 200. In the derivation cohort (n = 170), we measured PaO.sub.2/FiO.sub.2 with two levels of positive end-expiratory pressure (PEEP) ([greater than or equal to]5 and [greater than or equal to]10 cmH.sub.2O) and two levels of FiO.sub.2 ([greater than or equal to]0.5 and 1.0) at ARDS onset and 24 h later. Dependent upon PaO.sub.2 response, patients were reclassified into three groups: mild (PaO.sub.2/FiO.sub.2 > 200), moderate (PaO.sub.2/FiO.sub.2 101-200), and severe (PaO.sub.2/FiO.sub.2 [less than or equal to] 100) ARDS. The primary outcome measure was ICU mortality. The standard ventilatory setting that reached the highest significance difference in mortality among these categories was tested in a separate cohort (n = 282). Results The only standard ventilatory setting that identified the three PaO.sub.2/FiO.sub.2 risk categories in the derivation cohort was PEEP [greater than or equal to] 10 cmH.sub.2O and FiO.sub.2 [greater than or equal to] 0.5 at 24 h after ARDS onset (p = 0.0001). Using this ventilatory setting, patients in the validation cohort were reclassified as having mild ARDS (n = 47, mortality 17 %), moderate ARDS (n = 149, mortality 40.9 %), and severe ARDS (n = 86, mortality 58.1 %) (p = 0.00001). Conclusions Our method for assessing PaO.sub.2/FiO.sub.2 greatly improved risk stratification of ARDS and could be used for enrolling appropriate ARDS patients into therapeutic clinical trials., Author(s): Jesús Villar [sup.1] [sup.2] [sup.11], Lina Pérez-Méndez [sup.1] [sup.3], Jesús Blanco [sup.1] [sup.4], José Manuel Añón [sup.5], Lluís Blanch [sup.1] [sup.6], Javier Belda [sup.7], Antonio Santos-Bouza [sup.8], Rosa Lidia [...]