According to the Berlin definition, the acute respiratory distress syndrome (ARDS) is classified as mild, moderate, and severe by using an arterial partial pressure of oxygen (PaO2) to fraction of inspired oxygen (FIO2) threshold of 300, 200, and 100 mm Hg, respectively (1). Despite the above classification, a PaO2/FIO2 threshold of 150 mm Hg has been used to define severe ARDS and to determine eligibility in therapeutic randomized, controlled trials examining interventions, such as prone positioning (2) and neuromuscular blockade (3). In addition, the PaO2/FIO2 threshold of 150 mm Hg identified two separate subgroups of patients, namely, a moderate–severe (100, PaO2/FIO2, 150 mm Hg) and a mild–moderate (150< PaO2/FIO2< 200 mm Hg) ARDS subgroup, with different anatomical and physiological characteristics, according to a recently published retrospective study (4). The latter study, which included 105 patients with moderate ARDS, did not have the statistical power to examine whether the two subgroups have different clinical outcomes (4). Therefore, the clinical significance of the PaO2/FIO2 threshold of 150 mm Hg remains unexplored. We explored the clinical significance of the above-mentioned threshold by examining whether patients with moderate–severe ARDS have worse clinical outcomes than patients with mild–moderate ARDS.