Continuous measurement of mixed venous oxygen saturation (Sv-O2) has recently been introduced as a monitoring and management technique in critical care patients. To determine the impact of Sv-O2 monitoring on patient management and cost-effectiveness, we conducted a prospective, randomized clinical trial of 99 consecutive patients receiving pulmonary artery (PA) catheters in the ICU. One group (n = 49) received an Edwards quadruple-lumen flow-directed PA catheter. Another group (n = 50) received the Oximetrix Opticath, a flow-directed PA catheter utilizing reflective fiberoptic oximetry for continuous Sv-O2 monitoring. Data recorded included APACHE and TISS scores, duration of stay, number of catheter days, incidence of catheter problems and changes, number of blood gases (arterial and venous), and ICU mortality. Additionally, the early warning capability of Sv-O2 monitoring was analyzed by comparing the severity and outcome of potentially adverse hemodynamic events in both groups. Statistical analysis by step-deletion multiple regression analysis and the unpaired Student's t-test demonstrated that use of the Opticath was not associated with a decrease in potentially adverse hemodynamic events, length of ICU stay, or mortality. In terms of cost-effectiveness, mixed venous oximetry is economically favorable in a charge-based reimbursement system, but economically unfavorable in a fixed-cost-based reimbursement system. Although continuous Sv-O2 monitoring has been shown to be accurate and reliable, it does not appear to be beneficial in all patients requiring PA catheterization.