The incidence of pneumocephalus and factors contributing to its occurrence were studied retrospectively in 100 consecutive patients who underwent posterior fossa or cervical cord surgery in the sitting, park-bench, and prone positions. Supine skull x-ray films taken immediately postoperatively were used to determine the presence of intracranial air. Surgery in the sitting position uniformly resulted in pneumocephalus (32/32 patients). Pneumocephalus also occurred frequently among patients in the park-bench (29/40) and prone positions (16/28). Intraventricular air occurred more frequently when patients were in the sitting position (25/32) than in the two other surgical positions (3/29 and 4/16, respectively, P < 0.001). The high incidence of pneumocephalus and of intraventricular air that occurred when patients were in the sitting position is attributed to the large amount of cerebrospinal fluid drained due to gravitational effect. When surgical position is considered one of the contributing factors, only position significantly affected the frequencies of pneumocephalus and intraventricular air accumulation. None of the 77 patients with pneumocephalus suffered neurologic change related to the presence of intracranial air. We conclude the following: pneumocephalus commonly occurs after posterior fossa or cenvical cord surgeries, particularly when the surgery is performed in the sitting position; neurologic change caused by pneumocephalus is a rare complication after posterior fossa craniotomy; when a patient with coexisting hydrocephalus undergoes surgery, if the patient is in the sitting position, there is an increased risk of trapping a large amount of intracranial air.