Iatrogenic brain herniation, with meninges (meningoencephalocele) or without meninges (encephalocele), following mastoidectomy is rare. However, the seriousness of the complication and the potential for its prevention necessitate periodic review and emphasis. In this study, 6 patients with small iatrogenic dural injury during mastoidectomy, 1 patient with postoperative large encephalocele, and 2 patients with three spontaneous meningoencephaloceles are reviewed. These cases, and the literature, support the following: 1. A dural injury is necessary for herniation. 2. If the arachnoid remains intact, a meningoencephalocele results with the potential for cerebrospinal fluid leak into the middle ear and recurrent meningitis. 3. If the arachnoid is not intact, an encephalocele results which presents predominantly as a mass without a cerebrospinal fluid leak. 4. The dura and arachnoid may be torn during mastoidectomy, resulting in an immediate cerebrospinal fluid leak that can go easily unrecognized. Taking time to carefully inspect any area of dural exposure obviates this oversight. 5. A satisfactory technique of tegmen and dural-arachnoid repair in these ten cases is described.