Performing surgery with a patient in the head-down position for several hours may cause marked facial and conjunctival oedema. It is not known whether oedema might also be occurring within the cranial cavity. This study, using near-infrared regional cerebral oximetry, was performed to see if there was any evidence of cerebral anoxia during operations performed with patients in the head-down position compared with those performed on patients who were flat. Patients managed in the head-down position did not demonstrate any reduction in regional cerebral oxygenation, but those who were managed flat did. An explanation of these paradoxical findings is elusive.