A comparison of the fine structure of subdural neomembranes with the fine structural organization of the normal human dura-arachnoid interface discloses that neomembranes are not de novo proliferations of tissue from a smooth inner dural surface. Rather, a neomembrane is the result of proliferation and excessive thickening of the normal layer of dural border cells. On proliferation, the dural border cells form multilayered tiers and clusters of cells, transfixed by capillaries, with collagen fibrils and elastic fibers between them. Capillaries and collagen fibrils are absent from the normal interface layer. Pathogenetic concepts of chronic subdural hematoma need to be revised. Any pathologic condition inducing cleavage of tissue within the dural border layer at dura-arachnoid interface will be followed by proliferation of fural border cells with production of a neomembrane. There is no compelling reason to postulate that proliferation of the border cell layer is always secondary to traumatic hemorrhage.