Objective. Pulse-dye densitometry (PDD) could be a suitable, low-invasive alternative to thermodilution using a pulmonary artery catheter (PAC) for monitoring cardiac output. The aim of our study was to assess the reproducibility and validity of PDD compared to PAC-thermodilution. Methods. In 43 post-cardiac surgery patients, the mean of triplicate readings of cardiac output was assessed using both methods. In a subgroup of 26 patients, a second set of measurements was obtained on average 2 h later. Results. Reproducibility of consecutive measurements was slightly better for PAC-thermodilution than for PDD (median coefficient of variation of the triplicate measurements: 3.5% versus 5.4%, P < 0.01). Both methods correlated well (r = 0.84, p < 0.001). Using Bland and Altman analyses with PAC-thermodilution as the reference method, PDD showed a bias of −0.68 ± 0.82 L/min, mainly due to differences in higher ranges of cardiac output (>6.5 L/min). Measured changes in cardiac output were 81% concordant (i.e. <1 L/min different) between both methods. Conclusion. PDD correlates well with PAC-thermodilution and thus deserves consideration as a low-invasive alternative for measurement and follow-up of cardiac output.