The accuracy of a new method for measuring cardiac output with continuous wave Doppler technique from the cardiac apex (CWa) was studied in comparison with two conventional methods, pulsed Doppler technique using apical approach (PWa) and continuous wave Doppler technique using suprasternal approach (CWs). Study population consisted of 25 patients whose cardiac output had been determined by thermodilution method. None of the patients had aortic valve disease, significant tricuspid regurgitation or intracardiac shunt. Aortic flow velocity curves were recorded with the above three Doppler techniques and the cross-sectional area of the aortic root was measured by two-dimensional echocardiogram at mid-systole. Cardiac output (CO) by each Doppler technique was computed using the following equation: CO = aortic cross-sectional area x systolic velocity integral x heart rate. Cardiac output by CWa had better correlation with thermal output value than those by PWa and CWs (r = 0.94, r = 0.85, r = 0.59, respectively). Outputs by PWa and CWs tended to be underestimated compared with thermodilution output. Interobserver difference was smaller in CWa than in PWa and in CWs (4.5%, 7.7%, 6.6%, respectively). Thus, our new method would appear to be accurate and useful in estimating cardiac output, and so may be applied in its case of seriously ill patients.