Forty-four critically ill patients with or without adult respiratory distress syndrome (ARDS) were studied in an attempt to define critical levels of oxygen delivery. Blood lactate was used as the indicator of tissue hypoxia independent of cardiac output. Survival was good (55 percent) and blood lactate near normal for those with oxygen delivery more than 8 ml/kg/min. Below this level, survival was poor (14 percent) and blood lactate markedly increased. There were significant nonlinear correlations of lactate with O2 delivery (r = - .735, p less than .001) and cardiac output (r = - .602, p less than .001). Mixed venous oxygen was not a reliable indicator of blood lactate, survival, oxygen delivery, or oxygen consumption.