'Cystic papilloma' in humans? Demonstration of human papillomavirus in plantar epidermoid cysts. Fourteen cases of plantar epidermoid cyst were studied immunohistochemically and five of them were also studied electron microscopically for the presence of human papillomavirus. In eleven cases, the test results were positive for papillomavirus antigens, and in five of them, papillomaviruslike particles were observed by electron microscopy. Histological examination showed most of the cysts to contain intracytoplasmic eosinophilic bodies in cells of the cyst wall as well as the vacuolar structures in the keratinous mass within the cavity. These findings suggest an etiologic association between the papillomavirus and plantar epidermoid cysts. The "cystic papilloma" produced in rabbits by the Shope papillomavirus has been clearly demonstrated. We propose that human papillomaviruses play an important role in the genesis of plantar epidermoid cysts. This is the first report on the possible existence of a cystic papilloma in humans. 