Fifty unselected recovery room patients were monitored using a Nellcor N101 pulse oximeter. Non invasive Sa o 2 values were compared with simultaneous spectrophotometric measurements made on arterial blood samples (Co-oximeter). Mean pulse oximeter Sa o 2 was 86.9±6.08%, with extreme values of 68 and 95.5%. Arterial blood Sa o 2 was 91.25±6.28%, with extreme values of 66 and 100%. Linear regression analysis showed a good correlation between the two sets of measures (r=0.97; p <0.005). Twenty one patients had a Sa o 2 below 90% (mean : 84.83±5.93%); their measured arterial Sa o 2 was of 86.14±6.32%. The correlation between the two sets of low values was good (r=0.96; p <0.05). Pulse oximetry was a non invasive reliable method, but there were some limits to its clinical use. Hypothermia or shock vasoconstriction precluded the measurement of Sa o 2. Restlessness or shivering were responsible of probe dysfunction and false alarms. False Sa o 2 readings may result from high levels of carboxyhaemoglobin or dye infusion. Also, each disposable finger probe was reused for a mean of five patients, its cost being relatively high (170 FF, i.e. 28.3 US dollars).