Because of high mortality and morbidity, acute respiratory distress syndrome (ARDS) continues to be one of the clinical challenges for intensivists. The diagnostic criteria for ARDS published by the American-European consensus conference were over simplified and made it possible to conduct large-scale randomized controlled trails (RCTs). Thus, many RCTs have been conducted in attempts to evaluate new treatment modalities, but many have reported negative results, in part because this definition was too broad to be used as diagnostic criteria. Pulmonary ARDS and extrapulmonary ARDS differ in terms of their morbidity and mortality with the presence of organ failure being an important risk factor for mortality in ARDS patients. The Classification of ARDS into several subgroups, which take the underlying disease into account, might limit the number of patients enrolled in an RCT. Where as this subgroup classification would enable selection of a homogeneous population of ARDS patients and may be a key to conducting more focused RCTs and, therefore, having more reliable results.