Fifty patients were examined clinically and neurologically for seven days after pneumoencephalography. Headache was present in 78%, neck stiffness in 34%, pyrexia in 38%, vomiting in 34%, tachycardia in 74%, a change in the level of consciousness in 18%, and abnormal neurological signs in 30%. Of the 13 patients with epilepsy, there was an increased frequency of seizures in four, associated with increased EEG epileptiform activity in three. EEG abnormality either appeared or increased in 74% of cases on the second day after the air study. A mechanism for the production of these sequelae is proposed. It is concluded that these findings indicate that in most cases an organic brain syndrome follows pneumoencephalography.