Four cases of tension pneumocephalus after either posterior fossa craniotomy or translabyrinthine resection of acoustic neuroma with or without nitrous oxide anesthesia are described. Three of the operations were performed with the patient in the sitting position, and one was done with the patient in the lateral position. Of the three cases operated in the sitting position, no nitrous oxide was used at any time during anesthesia in one. Two patients failed to regain consciousness after the termination of anesthesia, and the other two developed the sudden onset of neurological symptoms 1 to 1.5 hours after the operation. In all cases computed tomography disclosed a large subdural collection of air. Re-exploration of the surgical wound or twist drill aspiration of the subdural air resulted in prompt recovery of neurological status in three patients, whereas the other patient's neurological status improved gradually without any specific treatment. The role played by nitrous oxide, the mechanisms by which air enters the intracranial space, the contributory factors, and the predisposing surgical conditions of tension pneumocephalus are reviewed and discussed. Dependent drainage of the cerebrospinal fluid, especially in a patient with coexisting hydrocephalus, seems to be the most important factor for the development of this complication.