OBJECTIVES: Bioreactance, the analysis of intrabeat variations in phase of a transthoracic voltage in response to an applied high frequency transthoracic current, was recently introduced for noninvasive cardiac output measurement (NICOM). We evaluated NICOM compared to thermo- dilution (TD) in several clinical settings. METHODS: 111 patients with a clinical indication for TD cardiac output (CO) measurement were recruited at five centers, including patients in cardiac catheterization laboratories, cardiac care units and intensive care units. CO measurements were made simulta- neously with TD and the bioreactance method and compared by regression analysis. RESULTS: For studies in the intensive care units, TD-based CO and NICOM were highly correlated (r = 0.78, P < 0.0001) and did not differ significantly from each other (P = 0.55). Results in the cardiac catheterization laboratory were similar (r = 0.71, P < 0.001; P = 0.28 NICOM versus TD). In subsets of patients, NICOM was shown to be better corre- lated with TD-CO than CO obtained with the Fick method or with standard bioimpedance-based measurements of CO. CONCLUSIONS: On average, compared to TD, bioreactance- based NICOM has acceptable accuracy in challenging clinical environments. Availability of such a tool may allow clinicians to have information about CO in patients where this information is not currently available to help diagnosis and guide therapy.