Since its introduction in 1970, the application for PA catheter monitoring has dramatically broadened. PA catheters provide the ability to obtain hemodynamic data for the assessment, monitoring, and therapeutic management of critically ill patients. Because of the potential complications associated with PA catheter monitoring, numerous editorials and articles have questioned the procedure's risk to benefit ratio. These articles address the insufficient availability of adequate outcome data or suggest no demonstrated benefit from PA monitoring. Subgroups of patients have been identified in whom the data obtained from PA monitoring altered the clinician's assessment and management. In spite of the study of Rao, which implies that PA catheters can improve mortality in critically ill patients, no scientific study of outcome has been able to confirm this impression. If an adequate understanding of the limitations of PA monitoring does not exist, appropriate selection and implementation of therapy cannot occur. The problem is not as much the technology, as it is the knowledge and expectations of the clinician. "Human" complications from inadequate understanding of the physiologic data are quite common. Reliance on the measured pressure is often misleading. The use of "absolute numbers" rather than trends or relative changes in the values monitored can compromise clinical assessment. Overzealous utilization and acceptance of any quantitative measurement without coordination with clinical judgment is fraught with failure.