Changes in bioimpedance during the cardiac cycle are inversely related to the volume of blood within the heart and great vessels. Estimating cardiac output from changes in the bioimpedance is a safe, painless, and noninvasive alternative which should be useful for repeated bedside determinations in individual patients. Cardiac output determined by the bioimpedance method using NCCOM3® (noninvasive continuous cardiac output monitor) was compared with that of the thermodilution method in 22 patients who underwent cardiac catheterization. Twenty-two patients, 7 males and 15 females ranging in age from 22 to 56 (mean 41.4), had various cardiac diseases. Coronary heart disease was present in 5 and valvular heart disease in 17. The cardiac rhythm was sinus rhythm in 4 and atrial fibrillation in 18. The results were as follows: 1) Bioimpedance provided a noninvasive, continuous and simple method to monitor cardiac output. 2) Cardiac output measured by bioimpedance and the thermodilution method correlated well (r=0.827 as a whole, r=0.902 in those with sinus rhythm). 3) Bioimpedance can be used in the clinical setting when the thermodilution method is not readily feasible and when continuous monitoring of cardiac output is indicated.