Extramammary Paget's disease (EPD) is a relatively common skin cancer wherein tumor cells have mucin in their cytoplasm. However, little is known about mucin expression in EPD. We examined immunohistochemically the expression of mucin core proteins (MUC1, MUC2, MUC5AC and MUC6) in 36 cases of EPD and found different patterns of expression in intraepithelial (n = 36), microinvasive (n = 13) and invasive lesions (n = 6). In normal skin, MUC1 was expressed in the sebaceous, eccrine and apocrine glands. MUC2, MUC5AC and MUC6 were not expressed in any of these. In the 36 intraepithelial lesions, MUC1 and MUC5AC were expressed in 35 and 36 lesions, respectively. MUC1 expression was also observed in all 13 microinvasive lesions and in all six invasive lesions. In contrast to the intraepithelial lesions, a decrease or loss of MUC5AC expression was observed in five out of 13 microinvasive lesions and in all six invasive lesions. MUC2 and MUC6 were not expressed in any of the EPD lesions examined. The combination of immunohistochemical staining for MUC1 and MUC5AC was useful for identifying invasive Paget cells. The decrease or loss of MUC5AC expression may have an important role in the invasive growth of Paget cells.