It has been suggested that in asymptomatic patients with leukaemias and very high white blood cell counts, the apparent hypoxaemia found using routine blood gas analysis is spurious, the result of excessive O2 metabolism by leukocytes. Pulse oximetry has been suggested as a means of overcoming the shortcomings of blood gas analysis in the assessment of these patients. We present the findings of two patients with extremely high white cell counts, which show that the hypoxaemia found is in fact true hypoxaemia, even in asymptomatic patients, and that met-haemoglobinaemia may be at least in part responsible for the low PaO2. We also showed that pulse oximetry was completely unreliable in our patients due to the elevated met-haemoglobin levels. We recommend that all patients with markedly elevated white cell counts should undergo blood gas analysis with no delay between sampling and processing and that patients with low PaO2 should undergo urgent cytoreduction.