The AA., after having examined the various hypothesis reported in literature about the etiopathogenesis and the contrasting anatomical and pathological data concerning the arachnoid supratentorial cysts, point out the remarkable frequency of they malformative and above-all post-traumatic genesis. On the formation mechanism of this last type, they agree upon the supposition expressed by Taveras and Ransohoff in 1953. The AA., therefore, after having analysed the principal morphological and topographical aspects, pay attention to the present diagnostic possibilities, above all in radiological range, where the TAC represents, by this time, the examination of election compared with traditional assurances with means of contrast. It follows the analysis of the personal casuistry, consisting in 6 arachnoid supratentorial cysts, two of which clearly post-traumatic and two, very probably, of malformative genesis (for the coexistence of data in favour of both suppositions). The AA. draw these conclusions: the CT Scan is the only diagnostic means which permits an exact pre-operating diagnosis on the nature of the lesion; the surgical and, above all, anatomo-pathological reports assume an essential rule for an exact etiopathogenetic interpretation of the lesion examined.