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The measurement of cardiac output by thoracic bioimpedance has been previously assessed in several studies. However, there continues to be disagreement as to whether this technique is sufficiently accurate for use in clinical practice or research. The current study aimed to compare thoracic bioimpedance (COTB) with thermodilution (COTD) in patients with stable chronic heart failure.A total of 282 paired measurements of cardiac output from 11 patients were analysed. There was good correlation between COTB and COTD (r=0.76, P<0.0001). However, Bland–Altman analysis revealed an average difference between values of 0.3 (2.2) l/min (P=0.02), suggesting a small average bias but marked variability in results. There was no significant correlation when results were expressed as percentage change from baseline and a significant average difference between values of 10.1 (30.1)%. There was no difference in between-day repeatability between thermodilution and thoracic bioimpedance [−0.2 (1.2) versus 0.1 (1.0) l/min, P=0.7].This study demonstrates a correlation between the techniques but shows a poor level of agreement. The method of COTB underestimated cardiac output compared with COTD, and this difference appeared greater with higher cardiac outputs. Agreement was worse when results were expressed as change from baseline. The present study does not support the use of thoracic bioimpedance in its current form as an alternative to thermodilution in stable patients with chronic heart failure.