Bioimpedance offers a simple non-invasive means of measuring systolic ejection volume and heart rate and thus cardiac output. Four pairs of electrodes are placed on precise locations on the chest and stroke volume is calculated according to the equation developed by Kubicek in 1966 and modified by Sramek in 1980. The aim of this work was to evaluate this method in patients with heart disease. In a series of 50 patients, the coefficient of correlation for cardiac index between impedance values (2.52 +/- 0.71 ml/min/m2) and thermodilution values (2.74 +/- 0.69 ml/min/m2) was 0.63 (p < 0.01). The mean difference was -0.2 l/min/m2 (confidence interval +1 l/min/m2 to -1.4 l/min/m2). There was no statistical correlation in patients with complete left bundle branch block, severe mitral or aortic regurgitation or dilatation of the aorta. In a group of 11 healthy volunteers, there was a good correlation between two measures taken at a 2 day interval (r = 0.95). The coefficient of variation ranged from 1.2 to 7% for ejection volume. Bioimpedance is reproducible and simple, authorizing its use for non-invasive monitoring of cardiac output in a given patient in various clinical situations.