Background. Pulse-contour analysis method (PCM) cardiac output (CO) monitors are increasingly used for CO monitoring during anaesthesia and in the critically ill. Very recently, several systems have been introduced that do not need calibration; among them the pressure recording analytical method (PRAM). Sparse data comparing the accuracy of the PRAM-CO with conventional thermodilution CO (ThD-CO) in cardiac surgery patients are available. Methods. In this prospective comparison study, paired CO measurements with a pulmonary artery catheter and a PRAM monitoring set were obtained 2030 min apart (t1, t2) in 23 extubated patients on the first postoperative day after cardiac surgery. Data were analysed by the BlandAltman method. Results. A total of 46 paired CO measurements (23 for each interval) were collected. The BlandAltman analysis showed a mean difference (bias) of 0.0 litre min-1 and limits of agreement (1.96 sd) of 4.53 to -4.54 litre min-1 [upper 95 confidence interval (CI), 3.265.80; lower 95 CI, -5.8 to -3.27]. The percentage error (1.96 sd/mean of the reference method) was 87. Conclusions. These results question the reliability of the PRAM technology for the determination of CO in postoperative cardiac surgery patients.