Paget's disease in an epidermal cyst.

A case of Paget's disease involving both the epidermis of the nipple and an adjacent epidermal cyst is reported. Immunohistochemistry showed similarity between the infiltrating cells in both sites and those of the underlying mammary adenocarcinoma. Epidermal cysts may be involved in a variety of disease processes which affect the epidermis in general and, unless they are subjected to careful histopathological examination, these diseases may be overlooked.