Cardiac output (QAo) can be estimated noninvasively by pulsed Doppler (PD) ultrasonographic determination of mean ascending aortic blood flow velocity (VAo) combined with M-mode echocardiographic determination of ascending aortic cross sectional area (AAo). Cardiac output is calculated from the volumetric flow equation (QAo) = (VAo) X (AAo). Pulsed Doppler measurements are known to correlate well with Fick and thermodilution methods; however, inter- and intraoperator variability of the velocity component of the PD method has not been determined in newborns. We did three repeated PD measures of mean aortic flow velocity in ten term infants (using four trained operators) to determine inter- and intraoperator reproducibility. The coefficient of variation for intraoperator variability (random error) for a single measurement of VAo was 11.7%. If three repeated measures by a single operator were averaged, the random error was 7.0%. There was little interoperator variability found.