A 44-year-old woman presenting with an inflammatory and palpable firm breast lump underwent surgical excision. Intraoperative frozen section analysis showed an extensive lesion consisting of ducts with intraluminal "necrosis". In addition, a very dense stromal inflammation was observed around these ducts, suggesting an invasive ductal carcinoma with predominant intraductal proliferation. However, on paraffin sections, epithelial cells close to the lymphocytic infiltrate were rare, subatrophic, without any neoplastic feature. The density and architecture of the lymphocytic infiltrate mimicked a breast lymphoma. However, immunochemistry and molecular biology investigation favored the diagnosis of a tumor-like lymphocytic mastitis. Although extremely rare, this particular form of lymphocytic mastitis, a diagnostic pitfall particularly during peroperative examination, should be recognized by pathologists.