Formalin-fixed, paraffin-embedded tissue sections from 45 patients with mammary and extramammary Paget's disease were stained immunohistochemically with the use of a polyclonal antiserum directed against a 14-amino acid segment of the c-erbB-2 oncoprotein. Positive membrane staining, which correlates with gene amplification, was found in 15 of 19 cases (79%) of mammary Paget's disease, 4 of 13 cases (31%) of vulvar Paget's disease, none of 8 cases of scrotal Paget's disease, and none of 5 cases of perianal Paget's disease. Of the 19 patients with mammary Paget's disease, specimens of underlying breast tissue were available from 14; all contained a concurrent ductal adenocarcinoma. Concordance of c-erbB-2 antigen staining between the underlying breast carcinoma and the pagetoid component was observed in 12 cases. Of the 13 patients with vulvar Paget's disease, 2 had superficial stromal invasion, and 3 had underlying, deeply invasive adenocarcinomas. One superficially invasive case was positive for c-erbB-2 expression. One additional case of vulvar Paget's disease had an associated primary pagetoid endocervical adenocarcinoma that spread into the endometrium; both the endocervical and vulvar components stained positively for the c-erbB-2 antigen. The results of this study indicate that the c-erbB-2 oncoprotein may play a role in the pathogenesis of extramammary Paget's disease. These results also suggest that the c-erbB-2 oncoprotein may function in vivo to promote intraepithelial spread of adenocarcinoma cells.