Authors performed comparative measurements of cardiac output using the impedance rheography and Fick's method in healthy men (6) reaching the value of correlation coefficient 0.88 and in patients with acquired cardiac defects (n = 21; r = 0.68). Authors also compared cardiac output values measured at rest and during exercise by means of thermodilution and rheographic methods in patients with heart failure (n = 9). Correlation coefficient was respectively 0.92 and 0.81.