We compared measurements obtained with a pulse oximeter (SpO2) against values obtained from a CO-oximeter in a patient with carbon monoxide poisoning. SpO2 was equal to the sum of the oxyhaemoglobin (HbO) and carboxyhaemoglobin (HbCO) values over the range of HbCO from 30 to 1%. This confirms the experimental findings that pulse oximeters measure HbCO as HbO. The patient was treated with oxygen (FlO2 = 50%) and recovered without any sequelae. Under these circumstances, the half-life of HbCO was approximately 2 h.