Park et al1 cite the Extracorporeal Life Support Organization (ELSO) registry and the CESAR (Conventional Treatment vs Extracorporeal Membrane Oxygenation in Severe ARDS) clinical trial as evidence for use of ECMO.4-6 The CESAR clinical trial cannot provide the credible and compelling evidence we need because CESAR lacked explicit methods. Explicit inclusion and exclusion criteria are not apparent in CESAR, and patient selection does not appear reproducible. The CESAR clinical trial was an effectiveness trial and should have, we believe, followed evidence of efficacy of ECMO. Unfortunately, a large observational case series, like the ELSO registry series, cannot provide the needed efficacy information. An efficacy randomized clinical trial that compares mechanical ventilation alone to mechanical ventilation plus ECMO, with well-defined and detailed rules for subject management, should clarify the role of ECMO.