The aim of the present study was to evaluate the validity of pulse oximetry screening for prehypoxaemia, to assess the agreement between pulse- and haem-oximetry and to elucidate any influence of peripheral temperature on pulse oximeter measurements. A consecutive prospective study was undertaken in 91 cardiac surgery patients still in treatment with controlled mechanical ventilation in the early postoperative period. We examined arterial oxygen tension (paO2), arterial oxygen saturation (SaO2) and pulse oximeter saturation (SpO2) from 657 arterial blood samples. The sensitivity of the pulse oximeter was 0.83, the specificity 0.73, and the diagnostic specificity was 0.10, at the chosen level of screening. The pulse oximeter showed a tendency to underestimate the oxygen saturation by 0.85%. The agreement between pulse- and haem-oxymetry was found to be good. The authors conclude that the pulse oximeter is acceptable for respiratory screening in postoperative cardiac surgery. The low specificity and the low diagnostic specificity results in frequent false alarms. Low peripheral temperature (down to 25%) do not influence the validity of either methods.