The epithelium of cholesteatoma covers a connective tissue, which is characterized by loose stroma, fibroblasts, histocytes, capillaries, and occasional mast cells. The migration theory, retraction theory, and metaplasia theory have been established and are considered to share the mechanism of genesis in the middle ear epithelium. Recently, some investigators have been interested in the role of keratin, Langerhans' cells, and mast cells in the disease process of cholesteatoma, but the role of the basement membrane has not been determined. Human cholesteatoma tissues were obtained from 9 patients during ear surgery, and these tissues were examined with light and electron microscopy to clarify the role of the basement membrane in the disease process of cholesteatoma. The results obtained were as follows : The epidermal cells of cholesteatoma contained a great amount of lysosome and phagosome in some places ; the basement membrane was discontinued in some places ; some of the epidermal cells protruded at those discontinuations, and organella(rER, Golgi apparatus) of cells markedly developed in the cytoplasm of the epidermal cell ; and the fibroblastic cells had exocytotic pits that surrounded the epidermal cells and made direct contact with the epidermal cells. These fibroblasts also showed evidence of morphological hyperfunction. The findings suggested that partial interruption of the basement membrane, as well as epidermal-mesenchymal interaction could contribute to the disease process of cholesteatoma.