In 20 children without shunts or valvular insufficiency, duplicate dye dilution and impedance cardiac outputs (ICO) were carried out. The duplicate dye dilutions had a standard deviation 0.259 L/min/m2, while duplicate ICO had a standard deviation 0.192 L/min/m2 (F = 1.82, p < 0.05). Of 53 sequential estimates, cardiac outputs measured by both indicator dye dilution and ICO had a 5.5% mean difference. In 21 subjects with left to right shunts, the ICO related well with pulmonary blood flow (r = 0.92) rather than systemic flow (r = 0.21). In 13 subjects with aortic insufficiency, sequential Fick and ICO had a 50% mean difference; the impedance measurement was found to be higher in every case. These data indicate that the impedance cardiograph can provide a noninvasive measure of cardiac output when there are no shunts or valvular insufficiencies. In subjects with left to right shunts the impedance cardiograph provides a measure of the pulmonary blood flow. When aortic insufficiency exists the impedance cardiograph is distorted such that it is consistently higher than Fick cardiac output.