In order to evaluate the relative effectiveness of various commonly used fluids including blood, colloids and crystalloids, the authors measured cardiorespiratory variables before, during, and after 400 therapeutic interventions in a wide variety of critically ill patients including 157 patients with adult respiratory distress syndrome (ARDS), and 54 patients who did not develop ARDS. The responses of ARDS patients were further stratified according to the time of their ARDS, i.e., during the period of their ARDS and either before they developed ARDS, or after they recovered and mechanical ventilation was discontinued.In essence, 500 ml of whole blood, packed red cells, colloids, 5% plasma protein fraction, and dextran-40, and 100 ml of 25% albumin produced significant improvement in hemodynamics and O2transport, while 1000 ml of crystalloids produced small but significant changes in arterial pressure and small statistically insignificant changes in flow, O2delivery, and O2consumption (Vo2).There were similar cardiorespiratory responses to the various agents in: (a) periods during ARDS, (b) not during ARDS, and (c) in non-ARDS patients. In the late or terminal stage, such as the last 48 hours of life, neither colloids nor crystalloids improved hemodynamic and oxygen transport variables; in these circumstances colloids and other fluid therapy may increase pulmonary shunting and worsen lung function.The authors conclude that in early ARDS, i.e., 2–5 days after onset of ARDS from trauma or sepsis, colloids, whole blood and packed cells improve cardiorespiratory function provided wedge pressures >18 mm Hg are not exceeded. Subsequently, an operational or empirical trial of therapy using cardiorespiratory measurements as criteria of efficacy is an appropriate approach.