The effect of extreme leukocytosis (>300,000/mm3) on the PaO2 of blood stored at 22° and 2°C was studied in two children with leukemia. Arterial blood samples drawn from these patients at 1 and 10 min were compared to blood samples drawn from eight control patients with normal leukocyte counts and PaO2 levels in the same range. One minute after drawing the samples, the PaO2 values at 2°C were significantly higher than those stored at 22°C. Later, there was a rapid and progressive decline in PaO2 values in both samples stored at 2° and 22°C. Rapid consumption of O2 by leukocytes is liable to result in erroneous diagnosis of severe hypoxemia in patients with extreme leukocytosis. Immediate cooling of the samples is insufficient to eliminate this process.