The amount of oxygen in mixed venous blood (SvO2) is a reliable indicator of the status of oxygenation of tissue in most clinical settings. Monitoring levels of SvO2 might, therefore, aid in managing critically ill patients. A recently developed adult pulmonary arterial catheter provides continuous measurement of SvO2 via fiberoptic bundles incorporated into the catheter, in addition to measuring hemodynamic pressures and cardiac output by thermodilution. We evaluated 100 consecutive fiberoptic catheters for accuracy and reliability of the system in 86 critically ill adult patients and determined the clinical usefulness that knowledge of SvO2 provided. Over a range of hemoglobin saturations from 24 to 85 percent, the correlation coefficient between in vivo SvO2 vs photometrically measured samples for 199 paired measurements was 0.95. Mean duration of usage for 33 catheters in the medical intensive care unit was 6.1 days; all catheters functioned normally until clinical conditions permitted removal. Continuous measurements of SvO2 detected unsuspected increases in tissue oxygen consumption during coughing spasms and positioning of the patient. In nine of 13 patients with hypoxemic respiratory failure requiring positive end-expiratory pressure (PEEP), we found a strong correlation (r = 0.88) between oxygen delivery and SvO2. Of the four patients not showing correlation, two had sepsis, and two had nearly normal values of SvO2 and oxygen delivery at all levels of PEEP studied. Continuous measurement of SvO2 improves monitoring of patients, facilitates titration of respiratory therapies, detects abrupt changes in tissue oxygen consumption, and identifies levels of PEEP associated with greatest oxygen delivery.