STUDY OBJECTIVE: To evaluate the accuracy of cardiac index (CI) as measured by echo-transesophageal Doppler monitoring (echo-TDM) with CI measured by the transpulmonary thermodilution technique. DESIGN: Prospective, observational study. SETTING: University hospital. PATIENTS: 16 patients scheduled for elective lung cancer resection. INTERVENTIONS: Patients underwent two-lung ventilation (TLV) and one-lung ventilation (OLV). MEASUREMENTS AND MAIN RESULTS: CI measurements were analyzed using Bland-Altman plots. Absolute values of CI as measured by both devices were highly correlated (r(2) ranging from 0.72 to 0.77), as were relative changes in CI after the start of OLV (r(2) = 0.48, P = 0.006). Before, during, and after OLV, TDM-CI biases were 0.46 +/- 0.28 L/min/m(2), 0.25 +/- 0.18 L/min/m(2), and 0.35 +/- 0.29 L/min/m(2), respectively. Limits of agreement remained stable throughout the three measurement periods (range -1.08 to 0.21 L/min/m(2)). The mean percentage error of CI measurements was 21.9% compared with the thermodilution technique. Although no adverse events were reported, 11% of measurement sets were incomplete due to poor signal detection. CONCLUSIONS: Echo-TDM is a safe technique, allowing continuous semi-invasive assessment of hemodynamic changes in most patients undergoing open-chest surgery. Doppler-derived CI values showed significant biases and moderate clinical agreement with transpulmonary thermodilution during TLV and OLV.