Study of neu-protein expression in mammary Paget's disease with and without underlying breast carcinoma and in extramammary Paget's disease. Correlation between neu/c-erbB-2/Her-2 gene amplification and overexpression of the neu gene product has been reported in tumors of glandular origin, especially ductal breast carcinomas. Formalin-fixed and dewaxed sections from 23 cases of mammary (MPD) and 9 cases of extramammary (EPD) Paget's disease were immunohistochemically stained by means of the monoclonal antibody 3B5 directed against an intracellular domain of the neu gene protein. All MPDs exhibited a distinct membrane staining of tumor cells independent of the presence of ductal breast carcinomas found in 18 cases. All these breast carcinomas also were positive for neu staining. In contrast to MPD, all EPDs were negative. Normal epidermis was always negative. Northern blot analysis sustained the immunohistologic findings in that the presence of neu mRNA could be demonstrated in two of three cases with MPD. Negativity in one case was due to dilution effects by nontumor cells. Our results suggest that Paget cells of mammary and extramammary localization, although very similar phenotypically, derive from different genetic accidents. Furthermore neu positivity in all MPDs and all underlying ductal carcinomas suggests common genetic alterations for both tumors. However the finding of five neu protein-positive MPDs without associated ductal breast carcinomas may suggest a somewhat different transformation process. 