Determination of estrogen receptors (ER) in breast carcinoma is valuable in the management of patients. However, little is known about the presence of these receptors in other tumors. Normal skin appendages and their neoplasms, including extramammary Paget's disease (EPD), might be expected to express ER since the breast is histogenetically related to sweat glands. In this study, 41 cases of skin appendage tumors (SAT) and 11 cases of EPD were stained using the ER-ICA monoclonal kit (Abbott, Chicago, IL) with a modified technique for paraffin-embedded sections. Controls included 10 biopsies of primary breast carcinoma and 4 cases of metastatic breast carcinoma to skin, all positive for ER. None of the samples of SAT or EPD showed staining for ER. Normal skin appendages were also negative. Normal vaginal epithelium in one case of EPD showed positive nuclear staining for ER. ER determination using immunohistochemical technique in paraffin-embedded sections may be useful in the differential diagnosis between malignant SAT and metastatic breast carcinoma in the skin. The absence of ER in normal skin appendages suggests that its apparition is a feature of specialized differentiation of breast epithelium.