In this clinical investigation 35 patients under mechanical ventilation were studied. It was possible to establish the precision of two pulse oximeters of different brands. The performance of these equipments was evaluated by comparing data with the hemo-oximeter and a statistical analysis employed the student t test. Results showed that bias between oximeters reading and hemo-oximeter was similar for both instruments. Eventually critical patients may present conditions that limit the use of this technique. The study showed that the discrepancy in the results observed may be attributed to the presence of the methemoglobin and possible to anaemia associated to hypoxia.