Although hyperbaric oxygen therapy has not been accepted as a standard therapy for traumatic brain injuries, it has been used, along with rehabilitative exercises, for traumatic brain injuries, and the standard protocol has a low risk of complications. We report a case of chronic traumatic brain injury that progressed to tension pneumocephalus after hyperbaric oxygen therapy. The patient was a 25-yr-old man who presented with left occipital bone fracture and subarachnoid and subdural hemorrhage after being hit by a car. He underwent craniectomy to remove the hematoma and cerebrospinal fluid diversion with a ventriculoperitoneal shunt for the treatment of hydrocephalus. Fifteen months after the trauma, the patient received hyperbaric oxygen therapy to promote functional recovery. Tension pneumocephalus developed after the first session of hyperbaric oxygen therapy, and immediate burr hole drainage followed by ligation of the ventriculoperitoneal shunt was performed. The patient's consciousness recovered gradually, and he was discharged home. We suggest that patients with unrepaired skull base fracture and cerebrospinal fluid diversion should be carefully evaluated before receiving hyperbaric oxygen therapy.