Twenty-one cases of Paget's disease have been studied using histochemical, ultrastructural, and immunohistochemical methods. Eight of the tumors involved the nipple, and 13 were extramammary (11 vulvar and two anal). The antibodies used were directed against different classes of cytokeratin proteins, epithelial membrane antigen, carcinoembryonic antigen, gross cystic disease fluid protein-15, and S-100 protein. The findings of this study provide conclusive evidence that Paget's cells, regardless of their location, are adenocarcinoma cells. Intracytoplasmic mucin is scanty in Paget's cells within the nipple, but typically plentiful in the extramammary sites where the cells are frequently signet-ring cells. The common mechanism for the evolution of Paget's disease is extension of cells from an underlying carcinoma, but the possibility that some cases, particularly in the vulva, develop from intraepithelial precursors cannot be excluded. Cancer 59:1173-1184, 1987.