Paget's disease of the nipple is a rare lesion nearly always associated with an underlying breast cancer, clinically impalpable and radiologically undetectable in about 40% of the patients. Fourty-four cases (28 mastectomies and 16 biopsies of the nipple) of Paget's disease of the nipple without clinically and radiologically detectable breast tumor were retrospectively studied by means of histochemistry and immunohistochemistry. Histochemical study showed that Paget cells were PAS positive and diastase resistant, and alcian blue positive at pH 2.5 in 32% and 18%, respectively. Immunohistochemical study showed that Paget cells were EMA and c-erbB-2 positive in 100% and 84%, respectively. Four of the six EMA positive and c-erbB-2 negative cases of Paget's disease of the nipple in which the underlying tumor could be pathologically analyzed were associated with ductal carcinoma in situ of cribriform or mixed types. These findings are helpful for differentiating Paget's disease from other lesions of the nipple, namely Bowen's disease and eczema which do not react with both antibodies, and from nipple adenoma which exhibits a positive staining with anti-EMA antibody and no reactivity with anti-c-erbB-2 antibody.