Abstract: In tlus paper, biopsies of fresh blister of 9 patients and adjacent normal-appearance skin of 5 patients were taken for electron microscopy. Whether the patients were generalized or localized type, the initial blister formation was found in the basal layer of the epidermis. Their characteristic ultrastructural changes were basal cell degeneration and cytolysis, especially in the subnuclear area of basal cell, in which clumping and aggregation of tonofilaments were seen, and the structure of organelles was obscure. In normal-appearance skin, the ultrastructure of epidermis was basically normal. It is authors' opinion that the pathogenesis of blister formation may be related to abnormal structure of tonofilaments.