Two patients are presented in whom cranial arachnoid cysts developed as diverticuli of the arachnoid membrane through small defects in the dura mater, eroded through the inner table, expanded within the diploe, and eroded the outer table of the skull. Based on observations at the time of surgery and the histological examination of these lesions, it is proposed that they are congenital in origin. Previously reported cases of "traumatic arachnoid cyst without fracture," "intradiploic cerebrospinal fluid fistula," and "middle fossa pitholes" appear to represent the same pathological process as the lesions reported in this paper. It is proposed that "intradiploic arachnoid cyst" is the most appropriate term by which these lesions should be described.