We report the second case of mammary Paget's disease arising in a supernumerary nipple of a 29-year-old woman. The epithelium of the nipple was infiltrated by large cells with abundant and pale-staining cytoplasm. The nuclei had a vesicular chromatin pattern and identifiable nucleoli. The cells were strongly immunoreactive with KL1, CEA and EMA, but did not show reactivity with PS100, HMB45, or erb-B2. The pathogenesis of Paget cells is unclear. In our case, the lesion showed nearly all the clinical, histological and histochemical characteristics of Paget's disease, though without involvement of mammary gland epithelium and underlying carcinoma. The possibility of an intraepidermal origin, either by transformation from epidermal keratinocytes or by derivation from intraepidermal precursor cells, has to be considered. The differential diagnosis against Toker cell hyperplasia is also discussed.