Blister formation in inflamed lesions of bullous pemphigoid were studied by electron microscopy. Three stages can be distinguished during the development of a blister: (1) There is marked edema in the dermis and epidermis and a rather pronounced cellular infiltrate in the dermis. Projections of basal cells are in contact with infiltrate cells. (2) The basal lamina shows discontinuities. Eosinophilic leukocytes migrate into the epidermis and degranulate. (3) In the fully developed blister the basal lamina is usually absent. Our findings in bullous pemphigoid are indistinguishable from those described in dermatitis herpetiformis.