Doppler echocardiography enables noninvasive determination of blood velocity and flow area through which quantitation of blood flow in vessels and across valvular orifices can be achieved. The stroke volume is rendered as the product of the flow area and the area beneath the velocity curve; on taking the heart rate into consideration, the cardiac output can be calculated. Essentially, this method can be used in the region of all four cardiac valves, the ascending aorta and the pulmonary artery. For calculation of the mitral and tricuspid velocity, the sample volume is positioned in the region of the tips of the leaflets or in the valve anulus. The flow area is most frequently calculated from the diameter of the valve anulus under the assumption of a circular cross-section. Additionally, in some studies, with respect to correction for area changes during diastole, separation of the leaflets in the M-mode echocardiogram has been employed. Determination of the right ventricular output is accomplished through the combination of the blood flow velocity in the pulmonary artery and the cross-sectional area of this vessel, the right ventricular outflow tract or the pulmonic anulus. To calculate flow in the ascending aorta, both pulsed and continuous-wave Doppler techniques have been employed and the diameter of the ascending aorta or the aortic root is derived echocardiographically. Comparative studies of the various methods show that measurement of flow in the region of the aortic anulus yields results somewhat superior to that of the other methods. Possible sources of error in these methods result from simplifying assumptions with respect to calculation of the area of flow, that is, equating the anatomical area with the area of flow, circular or elliptical cross-sectional models, temporal constancy of the areas as well as the velocities, that is, constant position of the sample volume, flat velocity profile and neglect of angle deviations.