The incidence of systemic air embolism during cardiopulmonary bypass is estimated to be 0.1%. However, the vast majority of instances are unreported and quietly ignored. The result may be disability or death. The control of air embolism obviously lies in prevention. The definite and specific treatment of this complication is hyperbaric oxygen. We report 6 patients referred to our institute because of air embolism during cardiopulmonary bypass. Of the 4 patients in whom hyperbaric oxygen therapy was delayed for 17 to 20 hours, 2 showed partial neurological improvement, as opposed to the success of hyperbaric oxygen therapy in the 2 patients in whom the delay was minimal. We conclude that as soon as the proposed open heart operation has been completed and there is an indication that air embolism has occurred, the patient should be treated with hyperbaric oxygen as quickly as possible, even before neurologic manifestations of cerebral ischemia appear.