The feasibility and the intrinsic variability of six different methods of echocardiographic and Doppler flow determination of cardiac output were analysed in 34 healthy volunteers. Four were excluded because of poor quality echocardiograms. The mean (range) age of the remaining 30 (12 women, 18 men) was 21 years (13-36 years). Cardiac output was calculated by six methods as a product of echocardiographically determined cross sectional area of the aorta (apical and suprasternal views), pulmonary trunk, tricuspid annulus, and mitral annulus (circular and corrected for diastolic variations), and the flow velocity integral measured by Doppler. Cardiac output ranged from 2.79 to 6.56 1/min (4.45 (1.29) 1/min) (mean (SD)). The feasibility of the methods ranged from 87% (26 patients) for the aorta from the suprasternal notch to 100% (30 patients) for the mitral orifice corrected for diastolic variations and for the tricuspid valve. The corresponding results for all 34 individuals were 76% and 88% respectively. Three way analysis of variance was performed in the 23 healthy volunteers in whom all six methods were feasible. Interobserver and intraobserver interpretative variabilities were 6.8% and 5.9% respectively. The intrinsic variability of each single measurement of cardiac output, independently of the observer and the method used, was 25%. Provided the image was suitable for analysis echocardiographic and Doppler flow determination of cardiac output was feasible in most healthy volunteers. But there was significant intrinsic variability for each of different methods. A single value of cardiac output in an individual should be interpreted with caution.