A detailed light, transmission, and scanning electron microscopic study of the arachnoid cyst wall was made in four cases and compared with that of the normal arachnoid mater in the human. Two hundred and eight reported cases of arachnoid cysts were analyzed to evaluate the anatomical distribution of these lesions and to get r an insight into their pathogenesis. The structural features of the arachnoid cyst wall that distinguish it from the normal arachnoid membrane are as follows: (1) splitting of the arachnoid membrane at the margin of the cyst, (2) a very thick layer of collagen in the cyst wall, (3) the absence of traversing trabecular processes within the cyst, and (4) the presence of hyperplastic arachnoid cells in the cyst wall, which presumably participate in collagen synthesis. The distribution of arachnoid cysts in two hundred and eight reported cases was as follows: Sylvian fissure, 49%; cerebellopontine angle, 11%; supracollicular area, 10%; the vermis, 9%; sellar and suprasellar area, 9%; interhemispheric fissure, 5%; cerebral convexity, 4%; and the clival and interpeduncular area, 3%. At each site, except possibly on the cerebral convexity, the cyst was associated with a normal subarachnoid cistern. This striking and nearly invariable association of arachnoid cysts with normal subarachnoid cisterns prompts the authors to hypothesize that arachnoid cysts represent a congenital anomaly of the developing subarachnoid cisterns in early intrauterine life. It is postulated that, during the process of the complex folding of the primitive neural tube and the formation of normal subarachnoid cisterns, an anomalous splitting of the arachnoid membrane occurs.