Development of the flow-directed pulmonary artery catheter in combination with reflective fiberoptic oximetry techniques allows the clinician to continuously measure mixed venous oxygen saturation (SvO2). A brief review of the determinants of oxygen balance, the Fick principle, and the technology of continuous SvO2 monitoring is preliminary to a debate between two clinicians on the usefulness of SvO2 monitoring. One clinician highly recommends use of the flow-directed pulmonary artery catheter in patients who require pulmonary artery catheterization. Monitoring of SvO2 is described as a safe, convenient, and reliable option that is cost-effective. Continuous availability of this dynamic physiologic monitor is of great value in improving understanding of serious disturbances in oxygen balance, providing information for rapid diagnosis, and guiding therapeutic interventions. Another clinician suggests a less enthusiastic approach to SvO2 monitoring and argues that SvO2 is a nonspecific index of the matching of oxygen delivery with supply. Although it is a useful adjunct in specific clinical situations, it provides uncertain information in the presence of a number of diseases. Major mistakes in patient management could follow from overreliance upon either absolute SvO2 measurements or analysis of trends over time. Use of the SvO2 monitor has not been proven cost-effective and may actually increase monitoring costs. Both clinicians agree that continuous SvO2 monitoring is valuable in many clinical circumstances, provided the limitations of the measurement are understood.