The authors report a previously undescribed small, well-demarcated breast tumor similar to a dermal cylindroma in a 63-year-old woman. The tumor was an incidental finding in a lumpectomy specimen for infiltrating lobular carcinoma. The cylindroma was surrounded by normal-appearing breast parenchyma and had the typical "jigsaw" pattern of epithelial basaloid islands. The islands showed focal squamous and myoepithelial differentiation. A notable number of reactive dendritic Langerhans cells permeated the epithelial cell islands, a feature considered to be characteristic of dermal cylindroma. There was also ductal differentiation. Thick bands of hyaline periodic acid-Schiff (PAS) stain and collagen IV-positive basement membrane material bordered the cell islands, and PAS-collagen IV-positive hyaline globules were seen within the cell islands. There was no nuclear pleomorphism or mitotic figures. The cylindroma did not express gross cystic disease fluid protein 15, carcinoembryonic antigen, estrogen and progesterone receptors, or cytokeratin 20 (CK20). There was diffuse and strong immunoreactivity to CK AE1/AE3, and focal reactivity for CK7 and smooth muscle actin. Cylindroma of the breast should be distinguished from adenoid cystic carcinoma and basal cell carcinoma. Although clearly epithelial, the exact histogenesis and cell phenotype of this unusual dermal type cylindroma of the breast are unknown.