Introduction: Pulmonary artery catheters carry significant risk and are widely used for thermodilution (TD) cardiac output (CO). A second generation noninvasive bioimpedance (Bio) continuous CO system is now available (BioZ, Cardiodynamics, San Diego, CA). CABG routinely utilizes TD and provides a challenging test for Bio since the use of retractors, median sternotomy and hemodilution may change the physics of Bio CO. Methods: Thirty-five patients undergoing primary CABG were studied with simultaneous (triplicate) measurements of TD and Bio. The technologies were compared at 4 time points (after induction, after median sternotomy before CPB, after CPB, after chest closure). Results: One hundred thirty-eight data pairs were compared and the overall Pearson's R correlation was R=0.758, P<0.0001. The best correlation was after induction (R=0.936, p<0.0001). The lowest was after chest closure (R=0.580, p<0.0005). With the chest open, after median sternotomy and immediately after CPB correlations were: .770 p<0.0001 and .742 p<0.0001 respectively. Bias and precision by Bland-Altman analysis were (-.283 and .618) respectively. Only 9 of 138 data (6.5%) points were outside 2 standard deviations by Bland-Altman analysis. Conclusions: Bio provides an alternative and continuous noninvasive way of following CO. It is uncertain why the correlations differed from induction to chest closure. However, at the time of chest closure, AV sequential pacing may have interfered with Bio signal for some data points as these made up the largest number beyond 2 standard deviations. In view of potential life threatening complications associated with TD, Bio provides a reliable, safe and more cost effective technique to assess CO.