Palmoplantar keratoderma (PPK) type Unna-Thost is known to be the most common form of a hereditary disorder of keratinization of palms and soles. The disease is clinically identical with PPK type Vörner which is histologically characterized by epidermolytic hyperkeratosis. By reinvestigation of the family originally seen by Thost in 1880, the features of epidermolytic hyperkeratosis were found histologically confirming the diagnosis PPK of Vörner. This proves the identity of PPK type Thost with PPK of Vörner. Because of the histological variability of epidermolytic hyperkeratosis, detailed light and electron microscopic studies are necessary in cases of diffuse types of PPK.