The article focuses on the issues regarding the reliability and validity of the acute respiratory distress syndrome (ARDS) definition which was discussed in the 1994 American-European Consensus Conference (AECC). Through a consensus process, a panel of experts developed the Berlin Definition, focusing on feasibility, reliability, validity evaluation of its performance. A draft definition proposed three mutually exclusive categories of ARDS based on degree of hypoxemia and four ancillary variables for severe ARDS which include radiographic severity, respiratory system compliance, positive end expiratory pressure, and corrected expired volume per minute. The draft Berlin definition was evaluated using patient level meta-analysis leading to the removal of ancillary variables.