This is the first documented report of a case of histiocytoid breast carcinoma in Japan. The patient was a 55-year-old woman with a right breast lump. An excisional biopsy revealed proliferation of histiocyte-like cells with slightly atypical nuclei within fibrous stroma. In places, tumor cells showed an invasive pattern similar to that in invasive lobular carcinoma including a targetoid pattern, as well as a focus of in situ lobular carcinoma with transition to the histiocytoid cells. Immunohistochemical stainings of the tumor cells demonstrated positive reactions for cytokeratins and epithelial membrane antigen. Gross cystic disease fluid protein 15, an apocrine marker, was almost uniformly positive in their cytoplasm. From these results, we supposed that histiocytoid breast carcinoma is a variant of invasive lobular carcinoma. Histiocytoid breast carcinoma is easily misdiagnosed as a benign lesion such as granular cell tumor or xanthoma. Points of differential diagnosis were described and we stressed that it is most important for pathologists to keep this variant in mind in order to avoid misdiagnosis.