The ultrastructure of epidermolysis bullosa simplex type Weber-Cockayne has as yet not been studied systematically. Therefore biopsies of blisters freshly produced by friction after a hot bath were investigated by electron microscopy in 8 subjects of 6 families. Blister formation was found always to start in the basal layer of the epidermis. At first the cytoplasm with the cell organelles appears to become diluted, then holes develop in the cytoplasm which merge and finally form cytolytic blisters between the dermo-epidermal junction and the nucleus. The cell organelles remain remarkably intact. The cause of blister formation is still unknown, however, no structural abnormality was observed.