Objective: The purpose of this study was to evaluate whether central venous oxygen saturation can be used as an alternative to mixed venous oxygen saturation in patients with cardiogenic and septic shock. Design: Prospective clinical study. Setting: A tertiary intensive care unit in a university hospital. Participants: Twenty patients with cardiogenic or septic shock requiring a pulmonary artery catheter and inotropic support. Interventions: None. Measurements and Main Results: The central venous oxygen saturation overestimated the mixed venous oxygen saturation by a mean bias (or an absolute difference) of 6.9%, and the 95% limits of agreement were large (-5.0% to 18.8%). The difference between central and mixed venous oxygen saturation appeared to be more significant when mixed venous oxygen saturation was