A 63-year-old patient with chronic lymphocytic leukemia presented with severe hypoxemia. However, the patient's hemoglobin saturation, measured by an ear oximeter, was normal. Although his WBC count was approximately 1,000,000/microliters, hypoxemia could not be explained by the consumption of oxygen by leukocytes. Therefore, arterial blood gas values were analyzed in both plasma as well as in whole blood. The PaO2 in the plasma was much higher than in whole blood and corresponded with the hemoglobin saturation measured by the ear oximeter. These findings suggest that very high leukocyte counts may interfere with the measurement of oxygen tension and that plasma may be used for blood gas analysis in this situation.