Yawning during hyperventilation occurred in certain patients post-thalamotomy. It was found that all of the lesions which elicited yawning (during the routine recording of electroencephalograms) were localized to the medial portion of the center-median nucleus. Yawning was noted to persist up to 3½ years post-surgery. Another group of patients who yawned when hyperventilated were patients with a history of a recent head injury who showed post-traumatic behavioral changes. Patients in both groups were young. There was no direct relationship between yawning and EEG abnormality. It was suggested that yawning during hyperventilation may serve as a sign of brain damage, especially at the brain stem level, in young patients.