Over-expression of the c-erbB-2 oncogene occurs in a proportion of human adenocarcinomas and in breast carcinoma is associated with poorer prognosis. Sections of formalin-fixed, paraffin-embedded tumour tissue from 22 patients with mammary and extramammary Paget's disease have been stained immunohistochemically using a monoclonal antibody (NCL-CB11) raised against a synthetic peptide from the C-terminal end of the predicted sequence of the c-erbB-2 protein product. All 12 cases of mammary Paget's disease showed membrane staining of intra-epidermal cells, indicating c-erbB-2 over-expression. Sections of underlying ductal breast carcinoma were available in nine cases; all nine tumours were c-erbB-2 positive and in eight the in situ component was of comedo or solid type. There was membrane staining of tumour cells in four of the 10 cases of extramammary Paget's disease; staining intensity was generally weaker than that observed in the cases of mammary disease. The possible implications of these findings for the histogenesis of both mammary and extramammary Paget's disease are discussed.