In most patients with ALI and ARDS, the clinician chooses a level of PEEP based on oxygenation. Typically, low oxygen saturation leads to an increase in PEEP, whereas elevated saturations prompt a reduced PEEP. One widely used approach is that devised by the ARDS Network, in which PEEP and Fio2 combinations are chosen from a table to achieve oxygenation goals.2 Basing the dose of PEEP on arterial oxygenation has multiple, serious drawbacks, however. First, the degree to which PEEP recruits lung varies dramatically from patient to patient. For example, extrapulmonary ARDS tends to respond to PEEP with higher compliance and CT scan evidence of recruitment, whereas pulmonary ARDS does not.7 Similarly, those with ARDS harbor more potential for recruitment than those with ALI, but, overall, the amount of recruitable lung is modest (13%).8 To the extent that PEEP is valuable because it prevents cyclic recruitment and derecruitment, a rational approach should base the level of PEEP on the degree of recruitability, rather than assuming all lungs behave identically.