A new method has been developed for the continuous measurement of aortic blood velocity in patients following cardiac surgery. Using an extractable Doppler ultrasound probe placed on the ascending aorta, the changes in aortic velocity were recorded up to 24 h postoperatively, in 14 patients undergoing coronary bypass surgery. Volume flow rate is calculated from the mean velocity, the diameter of the aorta and the angle between the ultrasound beam and the direction of the blood flow, by means of an analogue flow calculator. Estimation of aortic flow showed a correlation of r = 0.79 with cardiac output measured by a thermodilution technique. The main advantage of the system is that it allows continuous monitoring of cardiac output, as well as short and long-term trend analyses, during the early postoperative period.