To evaluate the accuracy of cardiac output measurements with biplane transesophageal Doppler echocardiography, we examined 26 sets of observations in 14 patients using thermodilution technique as the standard. A mitral inflow method by single-plane combined the time-velocity integral of mitral inflow at the mitral annulus with the area of mitral annulus, assuming it to be a circular shape, by use of either the four- or the two-chamber view. With both views, a mitral inflow method by biplane combined the average of time-velocity integral of mitral inflow from the four- and the two-chamber views with the area of mitral annulus, assuming it to be an ellipsoid shape. The correlation coefficients between thermodilution and single-plane method of cardiac output were 0.81 (SEE = 0.72 L/min) and 0.85 (SEE = 0.79 L/min) for the four- and the two-chamber views, respectively. The correlation coefficient with biplane method was 0.93 (SEE = 0.47 L/min). Thus, biplane transesophageal Doppler echocardiography can be used for more accurate estimation of cardiac output.