BACKGROUND Arterial pressure waveform analysis enables continuous, minimally invasive measurement of cardiac output. Haemodynamic instability compromises the reliability of the technique and a means of maintaining accurate measurement in this circumstance would be useful. OBJECTIVES To investigate the accuracy, precision and trending ability of arterial pressure waveform cardiac output obtained with FloTrac/Vigileo, versus pulmonary artery thermodilution in patients undergoing elective open abdominal aortic aneurysm repair. DESIGN A prospective observational study. SETTING Operating room in a university hospital. PATIENTS Twenty-two patients scheduled for elective, open abdominal aortic aneurysm repair. MAIN OUTCOME MEASURES Bias, limits of agreement and mean error as determined with Bland-Altman analysis between arterial waveform and thermodilution cardiac output assessment at four time points: after induction of anaesthesia (t(1)); after aortic cross-clamping (t(2)); after clamp release (t(3)); and after skin closure (t(4)). Trending ability from t(1) to t(2), t(2) to t(3) and t(3) to t(4), determined with four-quadrant and polar plot methodology. Clinically acceptable boundaries were defined in advance. RESULTS Bland-Altman analysis revealed a bias of 0.54 l min(-1) (thermodilution minus arterial waveform cardiac output) for pooled data, and 0.51 (t(1)), -0.42 (t(2)), 0.98 (t(3)) and 0.98 (t(4)) l min(-1) at the different time points. Limits of agreement (LOA) were [-3.0 to 4.0] (pooled), [-2.0 to 3.0] (t(1)), [-3.1 to 2.3] (t(2)), [-2.5 to 4.4] (t(3)) and [-1.7 to 3.7] (t(4)) l min(-1), resulting in mean errors of 58% (pooled), 45% (t(1)), 53% (t(2)), 52% (t(3)) and 41% (t(4)). Four-quadrant concordance was 65%. Polar plot analysis resulted in an angular bias of -12 degrees, with radial LOA of -60 degrees to 36 degrees. CONCLUSION Bias between arterial waveform and thermodilution cardiac output was within a predefined acceptable range, but the mean error was above the accepted range of 30%. Trending ability was poor. Arterial waveform and thermodilution cardiac outputs are, therefore, not interchangeable in patients undergoing open abdominal aortic aneurysm repair.