To date, the histogenesis of vulvar Paget's disease remains unresolved. New electron microscopic findings, in 2 of the 5 cases presented in this report, demonstrate that the Paget cell may differentiate along either squamous or secretory lines and thus be found in the epidermis or within the glandular adnexa. The prognosis of Paget's disease, when confined to the vulva, depends not on the site and extent of the epithelial involvement but on the presence or absence of subjacent carcinomata. While considered to represent a form of carcinoma in situ of the vulva, Paget's disease itself does not become invasive. Experience with topical 5-FU in the control of local recurrences after surgery is encouraging and offers a new approach to therapy.