✓ The structure of human arachnoid villi was investigated by light and electron microscopy with the aid of immunohistochemical techniques. The human arachnoid villi examined were basically composed of four portions: a fibrous capsule, an arachnoid cell layer, a cap cell cluster, and a central core. The arachnoid cell layer encompassing the central core was mostly covered by the thin fibrous capsule with an endothelial investment. However, the fibrous capsule was often absent at the apical portion of the villus and a factor VIII-related antigen stain failed to confirm the investment of endothelial cells. Instead, the arachnoid cell layer abutted directly upon the lumen of a lateral lacuna or the sinus. The arachnoid cell layer was thickened in places, forming cap cell clusters; it usually consisted of outer and inner zones. On vimentin staining, the former was slightly positive while the latter was strongly positive. The central core contained a network of arachnoid cells intermingled with connective tissue fibers and was in continuity with the cranial subarachnoid space. Electron microscopy showed that the arachnoid cells contained a larger number of intermediate filaments in the inner zone than the outer zone. Ultrastructural immunohistochemical localization showed that vimentin was localized at the intermediate filaments and desmosomal plaques of the arachnoid cells. The arachnoid cells showed a marked variety in both the cell forms and the number of intermediate filaments or desmosomes, depending on their location.