The USCOM (Ultrasonic Cardiac Output Monitors) device is a non-invasive cardiac output monitor, which utilises transaortic or transpulmonary Doppler flow tracing and valve area estimated using patient height to determine cardiac output. We evaluated USCOM against thermodilution cardiac outputs and transoesophageal echocardiography valve area measurements in 22 ASA PS4 cardiac surgical patients. Data collection commenced following pulmonary artery catheter insertion, with cardiac output measurements repeated after sternotomy closure. Failure to obtain transaortic Doppler readings using USCOM occurred in 5% of planned measurements. USCOM transaortic analysis was not planned for 11 patients with known aortic disease. Bias at the aortic window (n = 20) was -0.79 l/min with limits of agreement from -3.66 to 2.08 l/min. At the pulmonary window, failure to obtain Doppler readings occurred in 24% of planned measurements. Bias at the pulmonary window (n = 36) was -0.17 l/min with limits of agreement from -3.30 to 2.97 l/min. The USCOM estimates of valve area based on height showed poor correlation with the echocardiographic measurements of aortic and pulmonary valves (r = 0.57 and r = 0.17, respectively). It was concluded that USCOM showed poor agreement with thermodilution. The estimated valve area was identified as one source of error.