The CDI 400 blood gas monitor was evaluated by investigating its clinical performance in 30 patients undergoing cardiac surgery requiring cardiopulmonary bypass. Arterial and venous blood samples were withdrawn during stable hypothermic conditions. Reference analysis was performed on the ABL4 blood gas analyzer. A total of 229 samples were included for calculations of bias and precision. Simple regression analysis was utilized to illustrate the relationship between the tested monitor and its reference. Results revealed an acceptable agreement for pH and arterial pO2 measurements. Venous pO2 bias and precision were poor (+0.76 +/- 1.07 kPa). Arterial and venous pCO2 were generally overestimated (+0.5 +/- 0.4 kPa and +0.2 +/- 0.4 kPa). Performance given as correlation coefficients indicated a similar pattern. Slopes and intercepts deviated from the line of identity for all parameters analysed. In summary: the CDI 400 is a valid instrument in guiding the perfusionist with crucial trend information. However, its general performance implies that a reference blood gas analyser is still needed.