The diagnosis of hidradenocarcinoma is difficult due to a combination of factors including inconsistent nomenclature/ classification, rarity of the neoplasm, and variable morphology of cells composing the neoplasm. Immunohistochemistry has not been previously performed on a series of hidradenocarcinomas. We evaluated six cases of hidradenocarcinoma histologically and immunohistochemically using antibodies to gross cystic disease fluid protein-15 (GCDFP-15), carcino-embryonic antigen (CEA), epithelial membrane antigen (EMA), S-100 protein, keratin AE1/3, cytokeratin 5/6, p53, bcl-1, bcl-2, and Ki67. Histology suggested concurrent eccrine and apocrine differentiation of the cases. Ki67 and p53 staining was strongly positive in five of six tumors. The neoplasms stained with antibodies to CEA, S-100 protein, GCDFP-15, EMA, bcl-1, and bcl-2 in no consistent pattern. All tumors studied stained positively for keratin AE1/3 and cytokeratin 5/6. In making the diagnosis of hidradenocarcinoma, it may be unnecessary to separate hidradenocarcinoma into eccrine and apocrine categories, and although Ki67 and p53 may be helpful, histological parameters remain paramount.