A non-invasive method for cardiac output determination (COD) based on ultrasound-Doppler technique was evaluated in patients with cardiac disease at rest and during exercise, including patients with heart transplants. The aortic blood flow velocity was measured with pulsed Doppler technique from the jugulum, placing the sample volume just above the aortic valve, and the area from a parasternal 2-D echocardiographic measurement of the aortic annulus diameter assuming a circular area. Cardiac output was calculated as the product of the systolic velocity integral, the aortic annulus area and the heart rate. A high correlation was found between this method and a simultaneously performed invasive cardiac output (COF) and stroke volume (SVF) determination by the direct Fick method (COD = 0.3 + 0.9 x COF, r = 0.96, SDres = 0.5 l min-1 and SVD = 3.9 + 0.92 x SVF, r = 0.94, SDres = 6.9 ml). However, looking just at the systolic velocity integral compared to stroke index determined with the Fick method we found a low correlation, especially in patients with heart transplants. We conclude that cardiac output can reliably be measured non-invasively with this method--also in patients with heart transplants. The systolic velocity integral alone can be used for assessing changes in stroke volume but for absolute values of stroke volume and stroke index flow area should also be determined.