Morphologic mimicry among human malignant neoplasms is a well-known phenomenon in surgical pathology; both undifferentiated and "committed" neoplasms may exhibit this trait. One particularly common group of histologic simulants includes ductal carcinomas of the breasts, the cutaneous appendages, and the salivary glands. One hundred three tumors in this structural cluster were analyzed microscopically and immunohistologically to codify points of potential pathologic similarity and difference. All the lesions were typified by irregularly permeative clusters and cords of atypical polygonal cells with variable luminal differentiation. A proportion of primary neoplasms in each site demonstrated in situ ductal components; in the absence of the latter elements, however, it was not possible to make topography-related morphologic distinctions among them. Immunostains for gross cystic disease fluid protein-15 (GCDFP-15), carcinoembryonic antigen, S100 protein, c-erbB-2 oncoprotein, estrogen receptor protein, and progesterone receptor protein also showed largely overlapping phenotypes in each of the three tumor categories, with selected exceptions. These differences were elucidated through paired chi 2 analysis and included a statistically significant infrequency of GCDFP-15 in eccrine sweat gland carcinomas, a paucity of carcinoembryonic antigen in breast cancers, and an absence of estrogen receptor protein in salivary duct carcinomas. Such findings may be useful in predefined differential diagnostic settings involving the distinction between primary and metastatic ductal cancers of the breasts, skin, and salivary glands. Nevertheless, because of the striking homologies between such tumors at structural and protein-synthetic levels of comparison, it is mandatory that all available clinicopathologic information be used in this context.