We recently managed a patient with diabetic ketoacidosis who had an exceptionally high arterial oxygen tension of 144 mm Hg while breathing room air. This was higher than the calculated alveolar oxygen tension and suggested the impossible situation of an uphill diffusion gradient. We were surprised to find this is a relatively common phenomenon (occurring in 25% of a retrospective study of similar patients). There are a number of probable mechanisms responsible for this observation, all of which reflect the severe metabolic aberration occurring in these patients.