The aim of this study was to compare cardiac catheterization (CATH) with 2D echo-Doppler (ED) in clinically evaluating the stroke volumes (SV) needed to calculate aortic and mitral regurgitant fractions (aortic and mitral SV for the ED method, thermodilution and angiographic SV for the CATH). As there is no 'gold standard' for this kind of measurement, only subjects without valvular regurgitation were considered. In these subjects, though the two SV measurements needed to calculate the regurgitant volume should have been identical, there was, in fact a difference due to the systematic and random errors of the methods. We calculated the mean value and the standard deviation of this difference in a series of patients without valvular regurgitation in order to obtain an estimate of both systematic and random errors. In 20 patients studied by ED a difference of 11.9 +/- 16.7 ml was found. In 36 patients studied by cardiac catheterization the difference was 19.6 +/- 20.1 ml. A significant systematic error was found for both ED and the invasive method; The transmitral SV tended to be larger than the aortic and the angiographic SV larger than that obtained by thermodilution. To try to determine the extent to which the random errors could be attributed to the reproducibility of the measurements, we carried out computer simulations. The SVs of 50 000 hypothetical patients were randomly generated and then attributed a random error calculated on the basis of the variability of the CATH (thermodilution 4%, angiography 10%) and the ED measurements (aortic annulus 6%, mitral annulus 18%, mitral time velocity integral 10%, aortic time velocity integral 8%).