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Background. Cardiac output by modelflow pulse contour method can be monitored quantitatively and continuously only after an initial calibration, to adapt the model to an individual patient. The modelflow method computes beat-to-beat cardiac output (COmf) from the radial artery pressure, by simulating a three-element model of aortic impedance with post-mortem data from human aortas. Methods. In our improved version of modelflow (COmfc) we adapted this model to a real time measure of the aortic cross-sectional area (CSA) of the descending aorta just above the diaphragm, measured by a new transoesophageal echo device (HemoSonic 100). COmf and COmfc were compared with thermodilution cardiac output (COtd) in 24 patients in the intensive care unit. Each thermodilution value was the mean of four measurements equally spread over the ventilatory cycle. Results. Least squares regression of COtd vs COmf gave y=1.09×[95% confidence interval (CI) 0.96–1.22], R2=0.15, and of COtd vs COmfc resulted in y=1.02×(95% CI 0.96–1.08), R2=0.69. The limits of agreement of the un-calibrated COmf were −3.53 to 2.79, bias=0.37 litre min−1 and of the diameter-calibrated method COmfc, −1.48 to 1.32, bias=−0.08 litre min−1. The coefficient of variation for the difference between methods decreased from 28 (un-calibrated) to 12% after diameter-calibration. Conclusions. After diameter-calibration, the improved modelflow pulse contour method reliably estimates cardiac output without the need of a calibration with thermodilution, leading to a less invasive cardiac output monitoring method.