BACKGOUND: The measurement of cardiac output (CO) is an important part of anesthetic practice in patients undergoing major surgery. The thermodilution method (TDM), using a pulmonary artery catheter, is still accepted as the standard procedure. However, the use of the method is associated with several limitations and severe complications. The use of a transesophageal Doppler ultrasound monitor (EDUM; Hemosonic 100(TM)) is less invasive and calculates CO from a simultaneous measurement of blood flow velocity and diameter of the descending aorta. The aim of this study was to compare the cardiac output measured by the use of the TDM and an EDUM. Methods:In 20 patients undergoing major abdominal surgery, CO and other hemodynamic profiles were measured simultaneously by the use of the TDM and an EDUM. RESULTS: The cardiac output, cardiac index, and the systemic vascular resistance monitored by both devices were highly correlated (correlation coefficients range from 0.63 to 0.88). Bland and Altman analysis showed a consistently negative mean bias for an EDUM and the TDM. This finding indicates an overestimation of the hemodynamic profile by the use of an EDUM. CONCLUSIONS: Although Bland and Altman analysis showed a significant bias, the use of an EDUM results in cardiac output, cardiac index, and total systemic vascular resistance measurements that are considerably similar to those obtained using the TDM, and a strong correlation exists for the use of the two methods.