Measurement of cardiac output is an integral part of patient management in the intensive care unit. FloTrac/Vigileo is a continuous cardiac output monitoring device that does not need re-calibration. However its reliability has been questioned in some studies, especially involving surgical patients. In this study, we evaluated the comparability of FloTrac/Vigileo and transthoracic Doppler echocardiography in 53 critically ill patients requiring continuous cardiac output monitoring. Most of these patients had septic or cardiogenic shock. Cardiac output was measured by both FloTrac/Vigileo and transthoracic Doppler echocardiography. The bias and precision (mean and SD) between the two devices was 0.35 +/- 1.35 l/minute. The limits of agreement were -2.3 to 3.0 l/minute (%error = 49.3%). When patients with irregular heart rhythms and aortic stenosis were excluded, the bias and precision was 0.02 +/- 0.80 l/minute (n = 42). The limits of agreement were -1.55 to 1.59 l/minute (%error = 29.5%). Patient demographics (body surface area, gender and age) did not affect the bias, but there was a mild tendency for FloTrac/ Vigileo to register a higher cardiac output at high heart rates. Changes in cardiac output for two consecutive days correlated well between the two methods (r = 0.86; P < 0.001). In summary, with the exceptions of patients with irregular heart rhythms and significant aortic stenosis, FloTrac/Vigileo is clinically comparable to transthoracic Doppler echocardiography in cardiac output measurements in critically ill patients.