The aim of the study was to compare the standard technique of cardiac output determination by pulmonary artery catheter thermodilution (PAC-TD) with a noninvasive ultrasound Doppler monitor (USCOM Pty., Ltd., Coffs Harbour, Australia) in surgery for liver transplantation. We wished to determine if the degree of accuracy would allow the ultrasound cardiac output monitor (USCOM) to be used as an alternative monitor in a clinical setting in which wide fluctuations in cardiac output could be expected. This was a prospective method comparison study, with 71 paired measurements obtained in 12 patients undergoing liver transplantation in a university teaching hospital. Bland-Altman analysis of the 2 techniques showed a bias of 0.39 L/minute, with the USCOM cardiac output lower compared with that of PAC-TD. The bias was small and did not vary with the magnitude of the cardiac output. The 95% limits of agreement were Ϫ1.47 and 2.25 L/minute. There was good repeatability for USCOM measurements, with a repeatability coefficient of 0.43 for USCOM versus 0.77 for PAC-TD. We conclude that USCOM is acceptable for the clinical determination of noninvasive cardiac output, particularly in situations in which tracking changes over time is more important than knowing the precise value. However, the utility of USCOM is limited by its inability to measure pulmonary artery pressure.