Delayed post-traumatic erosion of the skull base is reported in three patients who presented as adults with cerebrospinal fluid fistulae and a history of recurrent meningitis. These skull defects were associated with herniation of the subarachnoid space into the diploe of the skull base, the paranasal sinuses and the orbit. This rare complication of head injury is assumed to have occurred as the result of a dural tear at the time of trauma. Its site probably determines whether a resulting meningocele widens the intradiploic space or broaches the cranial floor.