Epidermolytic hyperkeratosis (EHK) is a congenital autosomal dominant ichthyosis. The disorder is characterized by blistering, especially at birth and during childhood, and hyperkeratosis. EHK presents striking clinical heterogeneity, particularly regarding extent of body surface involvement, quality of scale, presence or absence of erythroderma, and palmar/plantar involvement. Histological examination of hematoxylin-eosin stained sections of EHK skin show a tremendously thickened stratum corneum and vacuolar degeneration of the upper epidermis, leading to the term "epidermolytic hyperkeratosis". Genetic studies have revealed that defects in either keratin 1 or keratin 10 are responsible for the disorder in several patients, and provide a means for prenatal diagnosis in at-risk pregnancies in these families. An understanding of the relationship of keratin structure to its function in vivo in human epidermis may lead to the development of novel therapies for EHK.