The authors attempted to simultaneously measure cardiac output by thermodilution (COtd), thoracic bioimpedance (CObi), and suprasternal Doppler ultrasound (COdopp) in 68 patients. Subgroups separately compared included patients whose lungs were mechanically ventilated, patients undergoing cardiac surgery, aortic surgery, patients with dysrhythmias, and patients with sepsis. The authors also studied the value of the ventricular ejection time (VET) in evaluating the agreement of CObiand COdoppwith COtd. Simultaneous CObiand COtdwere available in a total of 56 patients (416 data sets) with an overall correlation coefficient r = 0.61, regression slope (m) of 0.52, intercept (y) of 2.46, and mean (CObi— COtd) difference (bias) of −0.67 ± 1.72 (SD) 1/min. Simultaneous COdoppand COtdwere available in 59 patients (446 data sets) with an overall r = 0.51, m of 0.53, y of 2.05, and bias of −0.79 ± 1.95 1/min. CObiagreed most closely with COtdin patients whose lungs were mechanically ventilated, who had not undergone cardiac or aortic surgery, and with VET difference < 40 ms (16 patients, 99 data sets; r = 0.74; m = 0.97; y = 0.15; bias = −0.02 ± 1.53 1/min). COdoppagreed most closely with COtdin patients whose lungs were mechanically ventilated, who had not undergone cardiac or aortic surgery, and in sinus rhythm with VET difference < 40 ms (10 patients, 45 data sets; r = 0.82; m = 0.98; y = −0.07; bias = −0.82 ± 1.03 1/min). VET by radial artery can help evaluate the reliability of CObiand COdopp.