Fifty-five patients with extramammary Paget's disease were the source of material for this study. Step-sections were done through most of the specimens. Clinical information, including follow-up, was obtained on 45 of the 55 patients. Extramammary Paget's disease could be divided histologically according to where Paget cells were found, namely: 1) wholly within the epidermis and the epithelial structures of adnexa, and the dermis; 3) within the epidermis, the epithelial structures of adnexa, and contiguous epithelia of other organs such as the genitourinary and gastrointestinal tracts. Our conclusions are that extramammary Paget's disease is more than one disease and in most instances begins in the epidermis as an adenocardinoma and extends from there into contiguous epithelium of hair follicles and eccrine sweat ducts. Uncommonly, Paget cells extend from the epidermis into the dermis and from there may metastasize. Rarely, extramammary Paget's disease results from direct extension into the skin of an adenocarcinoma in a contiguous organ such as the genitourinary or gastrointestinal tract.