Hidrotic ectodermal dysplasia (HED) was observed in a 49-year-old Japanese man. His clinical signs included alopecia, dystrophic nails and palmoplantar keratoderma, but his teeth, facial appearance and sweating were normal. In his family, 9 members in 5 generations were affected, suggesting that the disorder has an autosomal dominant mode of inheritance. Biopsy specimens of the hyperkeratotic lesions of the palm and sole were studied with an electron microscope. The most prominent feature was the increase of the number of desmosomal discs in the thickened stratum corneum, suggesting that hyperkeratosis observed in HED is due to the delayed desquamation of the stratum corneum.