To describe the natural history of adult respiratory distress syndrome (ARDS) from the circulatory viewpoint, we measured hemodynamic and oxygen transport variables in a series of 126 postoperative and 65 septic ARDS patients. All patients had hypoxemia unresponsive to conservative measures and required mechanical ventilation. Data from this series were compared with data from normal healthy volunteers and from postoperative patients who did not develop ARDS. In addition, the temporal sequence of cardiorespiratory changes in the ARDS patients was observed for a 48-h period before and after ARDS was diagnosed. Before the development of ARDS, both postoperative and septic patients exhibited hypovolemia, pulmonary vasoconstriction, suboptimal myocardial performance, and reduced oxygen delivery and consumption relative to the increased needs of the hypercatabolic state. Because these antecedent changes were greater in ARDS patients who died, their early correction (in addition to standard ventilatory support) may prevent the development of ARDS and decrease its mortality.