This study compares a derivative Fick technique using carbon dioxide (CO2) with the thermodilution pulmonary artery catheter (PAC), for determination of cardiac output (CO). Subjects were sedated, mechanically ventilated adults following elective cardiac surgery Microprocessor controlled deadspace activation and side-stream capnography in a ventilator circuit enabled calculation of CO (CO(CO2)) every four minutes. Thermodilution CO (CO(TD)) was performed as clinically indicated and at 20-minute intervals. Simultaneous CO(TD)/CO(CO2) pairs were recorded from time of admission to ICU for a minimum period of two hours for each patient. There were 358 CO(TD)/CO(CO2) pairs recorded from 41 patients. Cardiac output measurements ranged from 2. 7 to 10.6 l/min. The bias (Bland-Altman) was 0.050 l/min (95% CI -0.024 to 0.125 l/min). The 95% limits of agreement were -1.354 to 1.455 l/min. This simple, non-invasive partial-rebreathing technique is a valid alternative to thermodilution for cardiac output determination in sedated, mechanically ventilated patients. There are significant implications for improved safety, reduced complexity and reduced cost in anaesthesia and intensive care.