Brain tissue herniation into the mastoid cavity is an uncommon phenomenon ; nevertheless, it has been described since the earliest years of this century. Serious sequelae and central nervous system infection may result from inadequate management, but it has become relatively rare with improved antibiotic treatment and surgical technique. It might be caused by chronic otitis media, previous ear surgery, trauma and neoplasm, or spontaneously. Spontaneous brain herniation is extremely rare. It gives rise to cerebrospinal fluid otorrhea, progressive hearing loss, meningitis and other neurologic symptoms. Treatment depends on site and size of dura defect. We experienced one case of spontaneous brain herniation into the mastoid cavity, which we treated with surgery.