Leukemic patients with severe leukocytosis experience a spurious lowering of the arterial oxygen tension if the arterial blood samples are not immediately analyzed. Arterial oxygen tensions were measured in three patients with acute leukemia; actual oxygen saturation was simultaneously measured with an ear oximeter. Blood gases demonstrated significant arterial hypoxemia even with immediate blood gas analysis in all patients. The ear oximeter in two patients revealed a relatively normal oxygen saturation while there was a simultaneous decrease in the calculated oxygen saturation. With reduction of the leukocyte count following chemotherapy in one patient, the calculated arterial oxygen saturation and the ear oximetry reading approximated one another. The ear oximeter is of value in the assessment of the true oxygenation status of patients who have leukemia and severe leukocytosis.