Pneumocephalus or intracranial pneumatocele is sometimes encountered after fractures involving the sinuses and mastoids. Occasionally it occurs as a result of infection with gas-producing organisms or as a result of an intracranial tumor eroding into the sinuses. Most of the reported cases followed fractures. The mechanism of production after fracture involves a break in the wall of the skull, through which air can be forced either by increased external pressure or by a ball-valve action of a piece of tissue or bone. With compound fractures not involving the sinuses or middle ear, there is no simple mechanism whereby increase in the external pressure can initiate intracranial air collections. With coughing, sneezing, swallowing, and blowing the nose, the pressure in the sinuses and mastoids is momentarily increased and when fractures occur through these regions air may be forced through the fracture site into the cranial cavity. Types Pneumocephalus may be either extracranial or intracranial. Extracranial collections of air occur in the subaponeurotic space of the scalp. They follow a break in the outer wall of the frontal sinuses or mastoid air cells. With increased pressure, air can be forced out to form swelling in the loose tissues. Intracranial collections are the more common type, the air being located in the subarachnoid space, the subdural space, the brain, or the ventricles. Air in the subarachnoid space usually arises from fractures in the posterior ethmoidal and sphenoidal cells and is often accompanied by a meningitis. Subdural air usually follows fractures through the posterior walls of the frontal sinuses, as there is a large potential space in the frontal region. This type may be unilateral. Intracerebral collections are one of the more common types and may be associated with subdural air. The air may be in the brain substance or ventricular system. Air may be demonstrated in either the subarachnoid or ventricular spaces or both, depending on the extent of adhesions, the location of the fracture, and the amount of external pressure. Delayed Development In a number of the reported cases, and in the case to be recorded here, the pneumocephalus developed after a latent period of several days up to several months. In Dandy's series of cases reported in 1926, only 6 of 24 traumatic cases for which the time interval was stated showed the pneumocephalus before one week; in 3 more it was evident before a month. In 10 it developed during the fourth to sixth weeks and in 5 at later dates, specifically 2 at two months and 1 each at three months, seven months, and ten months. The usual interval before recognition is about one month. The reason for this latent period has not been determined. It is our impression that the immediate surrounding hemorrhage and edema may be a factor in preventing the passage of air into the subdural region. Also, during the first few weeks the patient is usually at bed rest.