The beat-by-beat non-invasive assessment of cardiac output (Q, litre·min -1 ) based on the arterial pulse pressure analysis called Modelflow® can be a very useful tool for quantifying the cardiovascular adjustments occurring in exercising humans. Q was measured in nine young subjects at rest and during steady-state cycling exercise performed at 50, 100, 150 and 200 W by using Modelflow® applied to the Portapres® non-invasive pulse wave (Q Modelflow ) and by means of the open-circuit acetylene uptake (Q C 2 H 2 ). Q values were correlated linearly ( r =0.784), but Bland–Altman analysis revealed that mean Q Modelflow -Q C 2 H 2 difference (bias) was equal to 1.83 litre·min -1 with an S.D. (precision) of 4.11 litre·min -1 , and 95% limits of agreement were relatively large, i.e. from -6.23 to +9.89 litre·min -1 . Q Modelflow values were then multiplied by individual calibrating factors obtained by dividing Q C 2 H 2 by Q Modelflow for each subject measured at 150 W to obtain corrected Q Modelflow (Q corrected ) values. Q corrected values were compared with the corresponding Q C 2 H 2 values, with values at 150 W ignored. Data were correlated linearly ( r =0.931) and were not significantly different. The bias and precision were found to be 0.24 litre·min -1 and 3.48 litre·min -1 respectively, and 95% limits of agreement ranged from -6.58 to +7.05 litre·min -1 . In conclusion, after correction by an independent method, Modelflow® was found to be a reliable and accurate procedure for measuring Q in humans at rest and exercise, and it can be proposed for routine purposes.