Measurement of cardiac output may improve hemodynamic management in donor care. Selected traditional and more recent methods to quantify cardiac output are reviewed. The accuracy or concordance of these newer methods when compared with thermodilution techniques that use a pulmonary artery catheter-the current reference standard-is discussed. Data directly comparing these systems for measuring cardiac output in the donor population are unavailable. However, data from groups of hemodynamically unstable patients favor selection of a measurement method that permits comparison (calibration) with a reference standard. A prospective comparison of all methods against the pulmonary artery catheter thermodilution technique among donors would provide the best data to resolve this clinical and potentially cost-important question.