Formerly, adult respiratory distress syndrome was the name, now it known as acute respiratory distress syndrome (ARDS). However, he was the first to outline the characteristic defining features of this condition. Clinical entity consists of dyspnea, cyanosis resistant to supplemental oxygen, and bilateral chest infiltrates on chest radiography. In view of its apparent similarity to the recently described respiratory distress syndrome observed in newborns, it was termed adult respiratory distress syndrome. An immense body of work has grown up around the study of this condition; however, until recently, a lack of diagnostic standardization confused efforts to accurately define the incidence and predisposing factors of this condition. Additionally, attempts at therapeutic trials were hindered by an inability to agree on a consistent definition of this syndrome. Standard definition of ARDS and a less severe illness, acute lung injury is based on (1) chest radiographic appearance, (2) the ratio of the partial pressure of oxygen in arterial blood to the percentage inhaled oxygen concentration, known as the PaO2/FiO2 ratio, and (3) assessment of the left atrial filling pressure, either by means of a wedged pulmonary artery catheter measurement or by clinical assessment. ARDS is considered to be present in the setting of bilateral infiltrates on a chest radiograph, a PaO2/FiO2 less than 200, and a left atrial filling pressure less than 18 mm Hg or no clinical or radiologic evidence of elevated left atrial pressure. Acute lung injury is defined similarly, with the difference being that the PaO2/FiO2 is less than 300. Unlike earlier definitions of ARDS, the PaO2/FiO2 is defined regardless of the level of positive end expiratory pressure.