We report 2 cases of arachnoid cysts, one with a retrocerebellar and the other with a left temporal localization, in which immunohistochemical studies had been conducted. The results of the immunohistochemistry on the presence of carcino-embryonic antigen (CEA) and glial fibrillary acidic protein (GFAP), and of the scanning- and transmission electron microscopy revealed the cyst lining to be identical to subdural neurothelium. Progesterone receptors were found in the nuclei of cells lining the cyst, which also suggests the similarity of the cyst lining to arachnoid granulations and meningiomas as derivatives of subdural neurothelium, which also possess progesterone receptors.