The ultrastructure of the dermo-epidermal junctional area in biopsy material from 5 patients suffering from lichen planus has been studied during treatment with steroids. Tannic acid was used on the tissue blocks in order to provide high and diversified contrast for electron microscopy. The changes observed in the basal cells consisted mainly of degeneration of the mitochondria and disorganization of the tonofilaments. No ultrastructural changes were seen in the basal lamina, apart from occasional gaps, whose significance for pigment incontinence in old lichen planus lesions is discussed. Cellular infiltration was seen in the dermis, while the ultrastructure of the collagen fibrils of the papillary layer was normal. It is suggested that degeneration of mitochondria with subsequent disturbance of the organization and fixation of tonofilaments are events in the pathogenesis of the condition.