Abstract: A method and apparatus are described for estimating the cardiac output of the heart of a patient. An indicator is injected as an impulse in the blood stream of the patient, whereafter a first signal corresponding with the concentration variation of the indicator is measured at a position at a distance downstream of the injection position. Said first signal is supplied to a processor unit for determining the cardiac output from said first signal and the injected amount of indicator. Further, a second signal at least substantially proportional to the cardiac output is measured in the same blood stream adjacent the measurement position for the first signal. Both said signals are stored in a memory. Thereafter, said second signal is normalized by a predetermined value of said second signal and the first signal is corrected by means of the normalized second signal. The processor unit now determines the cardiac output from the corrected first signal.