Aggregation of tonofilaments within epidermal keratinocytes is a characteristic histologic feature of epidermolytic hyperkeratosis including the generalized form known as bullous congenital ichthyosiform erythroderma. The histologic distribution and the keratin composition of the altered tonofilaments were investigated to determine whether the aggregation was specific to any particular keratin(s). Skin samples from seven patients and one mid-trimester fetus with generalized epidermolytic hyperkeratosis, and from one patient with a localized or "nevoid" form of epidermolytic hyperkeratosis, were analyzed by using various microscopical and immunocytochemical methods. A conjunctival sample and cultured epidermal keratinocytes from one patient with generalized epidermolytic hyperkeratosis were also examined by electron microscopy and immunocytochemistry. Ultrastructurally, tonofilament aggregates were distributed within the suprabasal stratified epithelial cell layers of the epidermis, of the infundibular part of outer root sheaths, and of the sebaceous ducts and sweat ducts, selectively following the known distribution pattern of keratins K1 and K10. The abnormal tonofilaments were not found in any other cutaneous epithelia, in conjunctival epithelium, or in cultured keratinocytes, where K1 and K10 are absent or only minimally expressed. Immunoelectron microscopy showed that among the keratins detected in suprabasal epidermolytic hyperkeratosis epidermis (K1/K5/K10/K14/K16), the aggregated tonofilaments predominantly expressed K1 and K10 rather than other keratins. These results suggest that the keratin filament abnormality in epidermolytic hyperkeratosis principally involves K1 and K10 and raise the question whether epidermolytic hyperkeratosis might be primarily a disorder of one or both of these keratins.