1. We have measured aortic flow in the ascending aorta in man with a Bach-Simpson BVM 202 blood velocity meter, and aortic root diameter by M-mode echocardiography, and thus derived beat-to-beat cardiac output (Q). 2. We tested the technique in 21 patients (53 comparisons) with various cardiovascular problems against a thermodilution method, and in four normal subjects at rest and two levels of exercise (50 and 100 W) against a nitrous oxide rebreathing method. 3. We obtained excellent overall correlation in a range of 0.5–10 litres/min ( r = 0.98, n = 77, s y,x = 0.48 litre/min), the formula for the least squares regression being: ( Q Doppler) = 0.95 ( Q Thermodilution/N 2 O) + 0.11 litre/min. 4. The Doppler signal is sufficiently noise-free to obtain maximum acceleration of flow from the first derivative of velocity.