Despite the similarity in clinical appearance, there is a significant difference in the prognosis between primary extramammary Paget's disease (EPD) and the pagetoid spread of underlying regional internal malignancy (secondary EPD, pagetoid phenomenon). Fifteen cases of primary EPD (11 carcinoma in situ and four invasive carcinoma), seven cases of secondary EPD (five colorectal adenocarcinoma and two urothelial carcinoma), and six cases of anal canal carcinoma were retrieved and analysed immunohistochemically using six kinds of monoclonal anticytokeratin antibodies. No expression of cytokeratins 1, 5, 10, 13 and 14 was observed in any cases examined in this study. All 15 cases of primary EPD had the immunophenotype cytokeratin (CK)7+/CK20-. CK20 expression was diffusely positive in six cases of secondary pagetoid spread (two urothelial carcinoma and four colorectal adenocarcinoma), and focally in one case (a colorectal adenocarcinoma). In anal canal carcinoma, three of six cases showed CK20 diffuse expression and the remaining three cases expressed CK20 focally. CK7 expression was observed in three of six cases of anal canal carcinoma and in two of five cases of secondary EPD associated with colorectal adenocarcinoma. The combination of CK7 and CK20 demonstrates these to be useful markers in distinguishing 'primary' EPD from a pagetoid spread of extracutaneous malignancies. Namely, immunophenotypes other than CK7+/CK20- in Paget cells suggest underlying regional internal malignancy.