The purpose of this study was to analyse the clinical agreement between cardiac output measurements, obtained using a newly available continuous thermodilution technique, and the conventional intermittent bolus technique. Twenty-four cardiac surgical patients were intra-operatively monitored using both techniques. Additionally, two different averaging modes for the continuous thermodilution technique, either the previous 6 min (group 1) or 3 min (group 2) were compared. The mean difference between the continuous thermodilution technique and the intermittent bolus technique (bias), were calculated at eight selected time points. These ranged from −0.09 to 0.42 litres min−1 in group 1 and from −0.02 to 0.18 litres min−1 in group 2. There were significant differences at any time point between the groups. The relative error between continuous thermodilution and intermittent bolus techniques was <15% for the majority of measurements and was significantly higher in group 1 compared with group 2 just prior to cardiopulmonary bypass. Thus, the continuous thermo-dilution technique produced a clinically acceptable level of accuracy compared with the intermittent bolus technique measurements, especially when using an averaging mode for the previous 3 min.