The management of carbon monoxide poisoning requires an accurate assessment of the extent of blood oxygenation. Measuring the fractional oxyhaemoglobin content by using co-oximetry gives a true picture of the oxygen-carrying capacity of blood in the presence of carboxhaemoglobin. The use of readings from pulse oximetry or a standard blood gas analyser is insufficient and can be misleading. We report on a case of carbon monoxide poisoning to illustrate this potential pitfall.